What is the ruling regarding a woman taking care of her husband's uncle, who is elderly and bedridden, and whose care entails changing his diapers and clothes and attending to all his needs, given that his wife is incapacitated and all his relatives are elderly?

If an elderly Sheikh has no family or men to care for him, there is no harm in a non-mahram woman taking charge of his affairs, provided she is careful not to look at or touch his awrah (private parts) — especially the severe awrah — except with a barrier. The woman will be rewarded for this if she undertakes it seeking Allah's pleasure.