So, your sister will only come to your wedding if you pay for a childminder to watch her children, her OH can sit outside the venue while you get married (I'd probably be fine with that depending on the type of venue - it's a free country and all that), sit at the top table at the reception and also have a role so he feels part of the family? Sorry, she is being hugely unfair to you. It does sound like it is probably because her OH is coercing her into making these demands so I do feel for her too, but there is no way I would accommodate all or any of her requests. It's just weird that he so badly has to be included. It's the kind of thing you might ask if you had a 5yo child, not an adult partner. The top table thing I wouldn't mind either but asking you to pay for a childminder is taking the absolute piss.
I'd probably say something like
"Yes, the whole thing has been stressful for us all. We would love to see you there, but as everything has already been arranged we can't create a role for OH now. We would love your daughters to be there too, but, having just paid for a wedding, we can't afford to pay for their childminder for the whole day. It would be lovely to see you there, but I would hate to upset you and OH any further by making you feel as if you have to come under conditions you aren't happy with. So I would completely understand if you feel you can't make it."