Is the presence of a young man with the family of his deceased maternal uncle in the same house, where there is no unmarriageable kinship between him and his uncle's wife, considered a sin upon him or upon them, given that she is completely covered in her clothing?

It is not permissible to enter upon a foreign woman unless a mahram is present, or something that negates privacy (khalwa) and ensures safety from temptation (fitna). It is suggested that you marry her or her daughter, if possible. If you are compelled to stay overnight with them, secure an independent section of the house for yourself, and ensure that entry is only in the presence of someone who negates privacy, with her adherence to the prescribed hijab. As for your uncle's money, it is a right of his heirs, and it is not permissible to dispose of it except with the consent of those among them who are adult and of sound judgment.