As mother of twin daughters who both have twins, I've experienced this from both sides. For some reason, this generation of young parents expect to be able to have overnight stops at friends and in hotels, unencumbered by children, and to be able to leave the children at midday and return to them same time next day. Unsurprisingly, this was something our generation rarely, if ever, did. Having the opportunity to get a break like this once or twice a year might be reasonable with twins - yes, it's hard work, but I don't recall anyone promising full-time childcare would be otherwise. The children are their responsibility and if you can offer to help for a few hours, they should take advantage of what you are able to do and not quibble about it.
It is easier to mind them in their own home, where all their equipment is, but not everyone can do that. When one of my daughters asked me to have her children overnight so she could stay in a hotel, they were dropped off for tea and I took them back at 11am the next day. Plenty of time for her to return from her hotel spa break and have an uninterrupted night's sleep. They stop over for a night at my invitation now they are that bit older, too.
Some grandparents can manage more than that, with more than one child. I do what I can manage and will be there in emergencies. You should set clear boundaries and times, and not have to justify why you can't manage more. She knows very well that looking after twins can be hard work - that's why she wants a break! Negotiating something with you is much better than not having that break at all.