To put it into a nutshell;
DPs father, a generally obnoxious man anyway, was very abusive both physically and emotionally to him throughout his childhood (he makes snide comments now too and can't EVER be wrong). I am learning this little by little. DPs mother obviously stood idly by, but acknowledges that her husband is and was awful. Now that I am seeing the full extent of it, I just don't want to see them. They drive me mad. All I can see when I see them is the young boy my DP was being verbally abused and beaten.
DP thinks it is his duty to see them regularly, which I have to respect. I don't want to, but he wants me too (when really I just want to tell them to go to hell).
YABU - see them, smile and support your DP
YANBU - draw a line in the sand and leave him to deal with it.