Why do I often have the feeling that my parents and siblings use me as a scapegoat?

Because it might be true, some families find a scapegoat, the black sheep, so they can blame them for the things that go wrong. It's much easier to place the blame somewhere else than to face the hard truths about themselves.

They probably see you as an easy target. As someone with this same problem my therapist had me set boundaries with my parents and sibling. It's hard to do but enabling them is worse.

I am really sorry to hear that. I think you should try to make a list of all the things that they blame you for. Read every thing in the list and check which of the blames are right and which are wrong. Be honest with yourself while doing so. Also try to think from their perspective when doing so. Based on that you can find out whether you are really wrong or they are just blaming it on you for no reason.

Firstly, consider the situations and events when you felt like a scapegoat for yiur family members. Then, go talk to them and tell them why and how you felt that way. It's possible that you family isn't doing it on purpose. At least by talking about it, you can all figure out how to resolve this issue

Because I allow them to. I feel that they can because they love me. My prefer they do it to me than to cause others unhappiness.

If you are feeling troubled by feelings that your family is causing you, the best thing to try to do is to speak with them about how you feel.