Is it permissible to contribute to the costs of a sister's marriage from the interest of money deposited in a bank, and if not permissible, how can this interest be disposed of without sin?

Depositing money in usurious banks is forbidden (haram (definition)). However, if no Islamic banks are available, it is permissible to deposit it out of necessity, provided one restricts it to a current account. The usurious profits must be disposed of by spending them on Muslim welfare projects or giving them to the poor and needy. It is not permissible to use these funds to prepare a sister for marriage unless she is poor and in need. In that case, she should be given only what she needs, because a poor person who possesses forbidden wealth is allowed to give it as charity to themselves and their dependents to the extent of their need. Otherwise, she should not be given from it, as it is not among the legitimate uses for forbidden wealth.