I've been going through a similar thing, I think the holidays just bring out the worst in families. Since my two were diagnosed, my sister (who has two young girls) and my brother's girlfriend (with one girl from a previous relationship and their one year old) seem to have closed ranks and shut me and my two ASD DSs out. My brother pretty much goes with what his girlfriend says - he just doesn't want any trouble generally, he's an amiable sort.
Never mind that they'd never hurt a fly and really adore their cousins, they haven't been invited round for a couple of years now, and any invitations seem to be met with comments like 'Oh, we would, but we're sooooo busy!'. I can understand busy, I am too, but I can make time for a visit every six months ffs!
Then there's the little critiques of my parenting - if DS2 is having a tantrum because a bit of grass got into his ice cream it's obviously because I spoil him, but if I don't let him stay up til 10pm (I know what a nightmare the next day will be if his routine is gone and he then can't sleep for hours) then I'm too strict (these were at my parents's house btw, the only place we still meet!).
It's all so stressful I'm just not sure if it's worth the hassle any more, but I really grieve for the relationship we used to have and it makes me feel really alone, so I understand what you're feeling :(