1. Do I have to accept the invitation to be a bridesmaid?

Technically, you don't, but in reality it's pretty hard to say no. If it's a financial issue, be up front about it and see what the bride-to-be says. If she really wants you in the wedding party, maybe she can help come up with a solution or will ask you to participate in another way, such as being a guest book attendant or a reader at the ceremony.