Is there still an obligation upon the questioner for mistakenly wearing his clothes during Umrah without completing it, and subsequently being a guardian for his sister’s marriage contract, knowing that he completed the Umrah later?

For a woman's marriage contract to be valid, it must be solemnized by her Muslim guardian, who is not in a state of ihram (definition) and not a non-performer of prayer, in the presence of two just witnesses. Based on this, your contract for your sister was not valid because you were in a state of ihram, had not completed your 'Umrah (definition), and were a non-performer of prayer, which invalidates your guardianship or agency. The presence and consent of your uncle do not validate the contract. Therefore, the marriage contract with the husband must be renewed in the presence of two just witnesses, bearing in mind that this renewal does not affect the lineage of the children.