Answer
(Fatwa: 318/B=67/tb=1431)
(1) Since nowadays it is very difficult to safeguard money at home from robbers and thieves, so only in compulsion one is allowed to deposit one’s money in bank for security. And it is unlawful to deposit money in bank for taking interest or increasing it, for it is initiative for interest.
(2) If some one withdraws money from bank with the intention of giving it to poor and needy ones without the intention of reward, it will not be called taking interest. However, if someone withdraws it for personal use then it is interest.
Yes, you can marry their daughters and participate in their marriages. But you should avoid eating the food of those who do the job of writing or checking interest based calculation.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband