I agree with those posting that you're taking a child's word at face value. I know you're angry and upset, but isn't it possible that your anger over other bullying at school and the feeling that the neighbors are cold could be making you very quick to assume the worst and to believe a kid's statement -- which might not convey the whole story? Can you step back from your emotions, now that you've vented, and ask yourself a few questions?
The neighbor kid may be completely making up what "mom said" or could simply have misunderstood something mom said that was quite different. Also, the "Susan can't come into our house" thing -- is that something you have only heard from these kids, or from Susan herself saying "I can't go inside there," or from the mom directly to you? There is a world of difference between hearing it from Susan or from these kids, and hearing it from the parent's own mouth, said directly to you.
Do you really want your child going into this family's home anyway? It sounds as if possibly you don't know the parents very well -- that alone would be reason for any parent to not want their child to play with neighbor kids inside the house. That applies whether a child is special needs or not. Have you, yourself, been inside their house? If not, I wouldn't let my kid go freely inside.
As your daughter gets older you are going to have to deal with both real and perceived discrimination and bullying. Your anger is perfectly normal and you need to express it. But you also can help your child navigate these things better if you eventually can either shrug it off -- maybe she needs to stop playing with these kids, or play with them only in groups with other kids there -- or approach other parents calmly to say, "I heard that Susan isn't allowed in your house, and I'm not sure if the kids have that right. Could we talk about it?" Parents won't learn anything from your anger, but they could possibly learn from your calm approach. You may not change minds, but you will be modeling calmness and the ability to walk away from people who cling to ignorance -- IF that is actually what's going on. You won't know until you get the full story, but right now you're too angry to talk to this mom, for sure.