Your courage in finally talking about this now is an important step in dealing with it. You didn’t get to have a choice about your first sexual experience and that is the issue. This, in and of itself, is important because this boundary-setting should have been yours to make.
I’d recommend individual counseling with someone experienced in helping adults recover from early sexual abuse in their family.
Being able to talk about it here is a very good start, and continuing this in therapy is the next right thing.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral