Yes, I mean like a DNA test, just to settle things, for once and for all. If that's the case, I guess it can't be done... :/

I don't know. I don't know enough about you or the situation to really make a full assessment, but I'm afraid that the only way I see is to really, really talk to someone outside the family who can provide (professional) help. I understand that it's going to be mighty hard, even almost impossible, but it may just be worth it. What about your friends? If you don't want to talk to someone alone, have one or two of your closest friends accompany you. It's normal to need support, encouragement. You don't have to do it alone.

Why did you tell them not to?