Oh, OP, how very upsetting.
I am shocked that your own sister feels able to be so blatant in her explanation as if such discrimination is normal.
Though judging by so many snug prim posts on this thread about ‘appropriate behaviour’ it is normal 
FFS your son is a nephew and cousin, a family member, with Down’s syndrome. Differently abled.
I would not be able to be honest in celebrating love and family, the spirit of a wedding, with people who had no love or respect for my own family. So if I did attend it would be on fake terms. And the hurt would be there anyway.
The damage is done now anyway. I would need to have a heart to heart with my sister about the hurt if exclusion around disability. Otherwise you are left pretending every is ok, for the rest of your lives.
Damn stupid people who want everything to look and be like a staged manicured constant Instagram feed. Where is the humanity? The warmth? Tolerance and understanding? Imagination?
It’s pretty insulting to their guests too if they think that they can’t adjust their parameters to include someone who clearly has SEN.
Whatever you do about the invitation OP, the issue is that your sister and her Ds have shown their feelings about your Ds.
So sorry you have this to deal with.