I know the PC thing to do is to find a middle ground, to try and maintain some kind of contact for the sake of the cousins.
BUT, tbh, the adults will soon pass their obnoxious and poisonous feelings towards your DS through their DC, so he'll get it not only from the adults, but from the kids too.
What would I do? Engineer a visit sans DC and have it out, once and for all with the sisters and the mum too - cos she is either joining in or tolerating this disgusting behaviour towards her GS, your DS. If it ruptures everything between you and your sisters/mum whatever, it's well deserved. How absolutely DARE they be so nasty about a little boy, their nephew? theirs own sisters DS?
FWIW, your DS does sound boisterous and energetic, and I daresay he may be a handful, but that's normal!!!! The way you describe the cousins, they sound a little too quiet.... mark my words, they will kick off and rebel in their teens, and we are talking HUGE problems.
You know your DS is fine, the school knows your DS is fine, he has friends of his own. Your nieces and nephews sound freakish tbh.
Stuff em, stick with your DS and your family, they are not worth your time. If you keep exposing your DS to their poison, it'll affect his self esteem and they are not worth that, no-one is.
If my family were this rude to my DS, I swear to God, there would be serious arse kicking. I am soooo on your behalf.