This whole situation is setting off my hinky meter like crazy. I would say we're in the red zone, hinky-wise.
Your sister obviously still exerts a lot of control over you and especially your mother, as evidenced by you giving her waaaaaay too much information in that text. Basically, in order to avoid her unpleasant behaviour (and who could blame you?) you put yourself in a position where she can easily say "No problems, I'll come around and look after DD while you visit Dad. She won't be funny with ME there" and leave you without any hope of beandip or deflection or anything because she knows the entire, full situation from the get-go. I know you think you're avoiding the escalation but really you're just giving her more ammo.
Also, your mother is not an 'audience' before whom she will behave nicely, because your mother is already one of her victims. So having her allowed to visit while your mother is sitting is no guarantee whatsoever that she will behave in a non-toxic way. From what you have said are you *absolutely certain* that your mother wouldn't be bullied into leaving her in your sister's care for a while when your mother is supposed to be watching her and then keeping that from you?
I would like to offer some (((hugs))) because I don't think that this is an abusive situation that is fully in the past. From what you have described of your extended family there is an extremely long term, deep rooted, extremely sick emotional structure that you have not fully escaped from. I am incredibly impressed that you have made it as far as you have, to be honest, because that kind of inter-generational scape-goating often does permanent damage. You are obviously are very strong person who has somehow managed to form right ideas about human relationships in the middle of a quagmire of toxicity, but I don't think your journey is over yet.
My suggestion is that you sit down with someone you trust, perhaps your DH, and decide exactly what behaviour you are going to tolerate, what you are not, and how you are going to avoid it. That way you will have the reinforcement that you need to help you keep knowing that really, no, it's NOT you, it's them.
As to that Uncle, I really would enjoy the opportunity to tell him exactly what I think of someone who would say that to a person on their wedding day. Please, don't give him another thought.