The Shar’ie verdict with regard to accepting gifts and invitation for meals is that the majority of the giver’s wealth must be Halaal. If the majority of the person’s income and wealth is Haraam, it will not be permissible to accept gifts and invitations for meals. In the enquired case, if their sole income or majority of their income is from the Bank or Haraam resources, it will not be allowed to accept anything from them, unless they clearly mention that it is from separate Halaal income. Hence, you should return their gifts to them or give them to the underprivileged people, if you are unable to return it to them.
Besides, try to make an excuse for not accepting their gifts and invitations for meals. Even better, bring it to their attention politely and with wisdom that working in a Bank is impermissible in Shariah and that is the reason for not accepting their gifts and meal invitations. A similar ruling will apply to the sum of money that you received from your sister towards your Hajj expenses; that for your peace of mind, it would be best for you to return it, albeit in the form of a gift.