Is what the questioner did, caring for her mother's blind, disabled uncle—whom she used to clean, change his diapers, and undress for washing—considered a righteous deed or a sin?

Helping the elderly and the needy carries a great reward, especially if the person is a relative, a memorizer of the Quran, and a former Imam. However, one should not undertake any action until they know Allah's ruling concerning it. It is not permissible to look at the 'awrah (private parts) of another person, except for spouses or a female slave with her master. The prohibition applies equally to a father, a brother, and other maharim (unmarriageable relatives), based on the Almighty's saying: "And tell the believing women to reduce [some] of their vision," and the Prophet's (peace be upon him) saying: "Avert your gaze." What you did is fundamentally impermissible, but if you did it because there was no other way to clean the elder, then there is no sin upon you, for necessities permit the forbidden, and dire need takes the place of necessity.