Do I really have to accept them as family?

I understand where you are coming from. It will never be easy to accept someone whom you know nothing about. Your parents have to understand that it's a lot harder for you to confide and have that comfort of calling them brother and sister. You need to talk to them about how you feel, in order for them to really captivate what you feel inside. Try to open up to them, it's always a Good thing. You can't feel closer to them without being their friend first.