Is it obligatory to return money to a sister if it was taken without her knowledge to fulfill a need?

You must seek forgiveness and repent for the sin of taking the money, with remorse and a firm resolve not to return to it. You must also return the money to your sister, even if through indirect means, should informing her directly lead to discord. This is based on the Prophet Muhammad's (peace be upon him) sayings: "On the hand is what it took until it returns it," and "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today before there is no dinar or dirham."