As already noted, much of Washington's early life was passed at the homes of his elder (half-) brothers, Lawrence and Augustine, who lived respectively at Mount Vernon and Wakefield. When Lawrence developed consumption, George was his travelling companion in a trip to Barbadoes, and from him, when he died of that disease, in 1752, came the bequest of Mount Vernon to "my loveing brother George." To Augustine, in the only letter now extant, Washington wrote, "The pleasure of your company at Mount Vernon always did, and always will afford me infinite satisfaction," and signed himself "your most affectionate brother." Surviving this brother, he left handsome bequests to all his children.

Samuel, the eldest of his own brothers, and his junior by but two years, though constantly corresponded with, was not a favorite. He seems to have had extravagant tendencies, variously indicated by five marriages, and by (perhaps as a consequence) pecuniary difficulties. In 1781, Washington wrote to another brother, "In God's name how did my brother Samuel get himself so enormously in debt?" Very quickly requests for loans followed, than which nothing was more irritating to Washington. Yet, though he replied that it would be "very inconvenient" to him, his ledger shows that at least two thousand dollars were advanced, and in a letter to this brother, on the danger of borrowing at interest, Washington wrote, "I do not make these observations on account of the money I purpose to lend you, because all I shall require is that you return the net sum when in your power, without interest." Better even than this, in his will Washington discharged the debt.

To the family of Samuel, Washington was equally helpful. For the eldest son he obtained an ensigncy, and "to save Thornton and you [Samuel] the expence of buying a horse to ride home on, I have lent him a mare." Two other sons he assumed all the expenses of, and showed an almost fatherly interest in them. He placed them at school, and when the lads proved somewhat unruly he wrote them long admonitory letters, which became stern when actual misconduct ensued, and when one of them ran away to Mount Vernon to escape a whipping, Washington himself prepared "to correct him, but he begged so earnestly and promised so faithfully that there should be no cause for complaint in the future, that I have suspended punishment." Later the two were sent to college, and in all cost Washington "near five thousand dollars."

An even greater trouble was their sister Harriot, whose care was assumed in 1785, and who was a member of Washington's household, with only a slight interruption, till her marriage in 1796. Her chief failing was "no disposition ... to be careful of her cloathes," which were "dabbed about in every hole and corner and her best things always in use," so that Washington said "she costs me enough!" To her uncle she wrote on one occasion, "How shall I apologise to my dear and Honor'd for intruding on his goodness so soon again, but being sensible for your kindness to me which I shall ever remember with the most heartfelt gratitude induces me to make known my wants. I have not had a pair of stays since I first came here: if you could let me have a pair I should be very much obleiged to you, and also a hat and a few other articles. I hope my dear Uncle will not think me extravagant for really I take as much care of my cloaths as I possibly can." Probably the expense that pleased him best in her case was that which he recorded in his ledger "By Miss Harriot Washington gave her to buy wedding clothes $100."

His second and favorite brother, John Augustine, who was four years his junior, Washington described as "the intimate companion of my youth and the friend of my ripened age." While the Virginia colonel was on the frontier, from 1754 to 1759, he left John in charge of all his business affairs, giving him a residence at and management of Mount Vernon. With this brother he constantly corresponded, addressing him as "Dear Jack," and writing in the most intimate and affectionate terms, not merely to him, but when John had taken unto himself a wife, to her, and to "the little ones," and signing himself "your loving brother." Visits between the two were frequent, and invitations for the same still more so, and in one letter, written during the most trying moment of the Revolution, Washington said, "God grant you all health and happiness. Nothing in this world could contribute so to mine as to be fixed among you." John died in 1787, and Washington wrote with simple but undisguised grief of the death of "my beloved brother."

The eldest son of this brother, Bushrod, was his favorite nephew, and Washington took much interest in his career, getting the lad admitted to study law with Judge James Wilson, in Philadelphia, and taking genuine pride in him when he became a lawyer and judge of repute. He made this nephew his travelling companion in the Western journey of 1784, and at other times not merely sent him money, but wrote him letters of advice, dwelling on the dangers that beset young men, though confessing that he was himself "not such a Stoic" as to expect too much of youthful blood. To Bushrod, also, he appealed on legal matters, adding, "You may think me an unprofitable applicant in asking opinions and requiring services of you without dousing my money, but pay day may come," and in this he was as good as his word, for in his will Washington left Bushrod, "partly in consideration of an intimation to his deceased father, while we were bachelors and he had kindly undertaken to superintend my Estates, during my military services in the former war between Great Britain and France, that if I should fall therein, Mt. Vernon ... should become his property," the home and "mansion-house farm," one share of the residuary estate, his private papers, and his library, and named him an executor of the instrument.

Of Washington's relations with his youngest brother, Charles, little can be learned. He was the last of his brothers to die, and Washington outlived him so short a time that he was named in his will, though only for a mere token of remembrance. "I add nothing to it because of the ample provision I have made for his issue." Of the children so mentioned, Washington was particularly fond of George Augustine Washington. As a mere lad he used his influence to procure for him an ensigncy in a Virginia regiment, and an appointment on Lafayette's staff. When in 1784 the young fellow was threatened with consumption, his uncle's purse supplied him with the funds by which he was enabled to travel, even while Washington wrote, "Poor fellow! his pursuit after health is, I fear, altogether fruitless." When better health came, and with it a renewal of a troth with a niece of Mrs. Washington's, the marriage was made possible by Washington appointing the young fellow his manager, and not merely did it take place at Mount Vernon, but the young couple took up their home there. More than this, that their outlook might be "more stable and pleasing," Washington promised them that on his death they should not be forgotten. When the disease again developed, Washington wrote his nephew in genuine anxiety, and ended his letter, "At all times and under all circumstances you and yours will possess my affectionate regards." Only a few days later the news of his nephew's death reached him, and he wrote his widow, "To you who so well know the affectionate regard I had for our departed friend, it is unnecessary to describe the sorrow with which I was afflicted at the news of his death." He asked her and her children "to return to your old habitation at Mount Vernon. You can go to no place where you can be more welcome, nor to any where you can live at less expence and trouble," an offer, he adds, "made to you with my whole heart." Furthermore, Washington served as executor, assumed the expense of educating one of the sons, and in his will left the two children part of the Mount Vernon estate, as well as other bequests, "on account of the affection I had for, and the obligation I was under to their father when living, who from his youth attached himself to my person, and followed my fortunes through the vicissitudes of the late Revolution, afterwards devoting his time for many years whilst my public employments rendered it impracticable for me to do it myself, thereby affording me essential services and always performing them in a manner the most filial and respectful."