I have a physical disability caused by a neurological birth defect, and I can well remember being incredibly self conscious about it at your DDs age. I had, and still have a really supportive circle of close friends, but I well remember the bullies too - and I’m including your MIL in that group, because that’s what she is. As I’ve got older I’ve realised that the majority of people are kind and inclusive, and I’m sure your DD will come to that realisation herself. But I’m still really shocked at some peoples’ attitude towards disability and I’m really sorry that one of the first experiences of that for your DD is from her own grandma - and at such a tender age when, as well as the normal pressures on teens, she will be coming to terms with her limitations and learning how to navigate them
Your DH is letting his daughter down badly IMO. She perceives that grandma sees her as inferior to her brother, and she clearly realises her dad is not speaking up for her when he should be. Added to that it seems that if she tries to defend herself she’s told not to be rude !! She can’t win can she ? If he won’t defend her himself, then he should be encouraging her to stand up for herself, not sending the message that having a disability means she has to keep quiet and put up with shit !!
I would tell DH that he has no right to tell you to stay out of it, she’s your DD too. The best way to tackle it would be together, but it doesn’t sound as though that’s going to happen.
If he won’t step up then I would sideline grandma at the first opportunity because I think she is having a hard time accepting that her GD is disabled - hence her asking repeatedly about a cure. I think you need to be firm in confirming that, as much as you wish it wasn’t the case, the condition is not curable, that DD will learn her own way of coping with the difficulties it poses, and that her disability doesn’t define her.
Let her know how much she’s upsetting both her GC with her insensitive and ableist behaviour - the comparison between DD and DS, the snide comments and inappropriate reactions to the effects of DD’s condition, need to stop.
Tell her how hurt your DD is and that she doesn’t want to see her, and neither does DS because he sees what’s going on - and that you are supporting them both in that, until such time as grandma has a change of attitude.
Grandma is a bully - that fact is evident by your DH’s reluctance to stand up to her, and also by the fact that any attempt to disagree with her results in a row. I don’t think you can avoid that row TBH, I think it’s just a matter of whether you want to delay having it until DD has had her confidence damaged, starts internalising the hurt she is feeling, and you have to add mental health problems to the difficulties she already has.