I think I may be going against the grain here.
So, after all these years of them not bothering with you, they need help, and you feel that because you have not been friends all these years, you should not help them out.
Look upon it this way. There is a little girl who is not invited to a wedding, who has a sister who is part of the wedding party. There is a grandmother who is trying to do her best by this little girl. And you are saying tough luck, stay in a hotel. The people you are inconveniencing the most is your husbands mother and her grandchild. Do you think they would have taken the step to ask to stay in your house if it was not for their daughter? Otherwise, a hotel would been best all round, possibly preferable to them!
Of course, it is not your problem. They are cheeky beggars, and dont deserve your hospitality, eh? But they are also family, and your mil wont be impressed with you. She will respect your decision, but she will most likely lose some of her respect for you. Is it worth it? Over some potential mess and a few dirty towels?
I think you are being a little petty and shortsighted.
If it was me, I would tell them they are welcome to stay in your house, but you like to come home to a clean house, and will expect it to be as spotless as you leave it. And to bring their own bedding and towels, as you will have a lot of laundry yourself after your weekend away. At the same time, take the opportunity to say you would like to invite them over for a time when you are at home, as it would be nice to spend some time with them.