If a couple split and say to everyone that it's amicable, they'd like to stay friends with each other (and current mutual friends, family etc) then that's great, makes it easy.
But when it clearly ISN'T an amicable split, you just can't be friends with both sides. Sorry, but it doesn't work and it's fucking hurtful.
When I split with my abusive Ex, I told friends some of the shit he put me through, and even after hearing that, a few of them thought they should keep in touch with him on FB etc 
.
As far as his family were concerned, I didn't contact them after we split. I never expected them to contact me, I entirely understood their loyalty was to their son, whatever I thought of him.
My bf is in a situation where his family have decided (for pretty pathetic reasons) to remain in contact with his Ex...he's tried to explain to them that he feels it's disloyal, and that he actually really needs their support, but his parents just gloss over his feelings, his DBs have splinters in their arses from taking the 'we're not getting involved' tack and his SIL (who is now best buddies with the Ex, despite her slating SIL constantly behind her back) now refuses to speak to BF or have anything to do with me (I'm not the OW, I met him a year after his Ex left him!). If they didn't live several hours away, I wouldn't put it past his SIL inviting his Ex to a party. The only difference in that case would be she wouldn't invite us (unless pressured by BF's mum).
I'm afraid I do think blood is thicker than water - I see how upset my Bf is by it all (his Ex isolated him from all his friends, so the only friends they had were her friends and their DHs...who have all cut him dead since the divorce - hence why his family are so important, but they can't see beyond 'keeping the peace').
Sorry for your shitty situation OP, I do think you're entirely right to be hurt. The only consolation is that your arse of an Ex is some other woman's problem now. Not that it makes up for the lack of support from your own family though.