Pre-empt it and tell them that your parents have paid for you to fly over, and you'd love to see them if they can get to you. Then stop the rest of the conversation with "we'd love to see you, but we won't be able to come to you." If SIL drives, could she drive your MIL down?
I'm taking it your parents live somewhere like Outer Hebrides or Lands End? If so, and you are feeling nice, you could book a round trip that flies into where your inlaws are, get an internal flight down or up to your parents and flies out of there. Find a travel agent online who will be able to find the cheapest way of doing it. I do understand that it will be difficult for someone in a wheelchair to travel.
However, we made the effort to go over to see my MIL in Ireland this year. I don't really like her, we are always nice to each other when we see her but I can only tolerate her in small amounts.
We booked a 10 day holiday 20 minutes down the road from where she lives and she saw us 5 times in that. Once when we went over (couldn't go again as her house is filthy and one of the kids had an allergic reaction). once when we met her in town and she went off with my husband for lunch, ignoring the children, Once when we took her out for lunch as a family, once when we took her and her husband out for dinner, and the last day when we invited her to come over so she could have all the food we had left and she asked us to pay for her petrol....
Sometimes people just aren't who you want them to be and I have made my peace with the fact that she will never be the loving and involved grandma that my mum is. My mum also lives 6 hours from us, so it isn't a question of distance.
Make the arrangement you're comfortable with, offer to see her on terms that suit you and then ignore any further comments and post what you want on FB