How should a girl act with her family who borrow an exorbitant usurious loan to hold a lavish wedding, and she shares accommodation and meals with them, knowing the impermissibility of usurious money, and should she repay this loan later?

Borrowing with usury is forbidden by scholarly consensus and is considered a major sin. Extravagance is a reprehensible characteristic, also forbidden.

Your duty is to advise your family gently and kindly, explaining the prohibition of usury and the grave sin of borrowing with it. If they repent and show remorse, their repentance is hoped to be accepted, and they must repay the usurious loan without the interest, if possible.

What was purchased with this loan has become their property, and there is no harm in benefiting from it, because the sin of the usurious loan pertains to the borrower's liability, not to the money itself.

Your participation in the wedding is permissible if it is free of objectionable acts, as is partaking in the food and drink. It is permissible for you to repay the usurious loan without the interest, unless there is harm in not repaying it.

If your refusal to participate with them has an impact on them abandoning usurious dealings, then you should refrain, while maintaining filial piety towards your mother and upholding kinship ties.