It took a long time before I shared because I had learned to NEVER share with anyone or it would be used against me.
First 2 years of therapy for me, I didn't feel like I could fully open up to the therapist for this reason. Oy. Now my poor therapist hears it ALL! My husband knows what's going on in my therapy sessions so he can be prepared if I exhibit some odd behavior for a day or two after my session. I don't hide my parents from anyone (though I may give them warning in advance). I also have finally stopped apologizing for them (largely thanks to this board!).
It can be very hard to break that family taboo of "not airing your dirty laundry".