What is the ruling on living in apartments built by one's father with money he earned from working in the documentary credits department of a bank, and is it permissible for him to help us with this money for marriage?

If your father deals with both permissible and impermissible letters of credit, then his salary is mixed, and it is permissible to accept his gift, though it is disliked (makrooh). However, if his work is exclusively in the impermissible or predominantly so, then it is not permissible to accept his gift or present; because in this case, it is difficult to be safe from unlawful money. And whoever abandons something for the sake of Allah, Allah will compensate him with something better.