My DM lives on her own, she moved to the same city we live in when she left her husband, to be close to us. She works part time in an 'essential worker' role, dealing face to face with the public and I've been shielding as I'm CEV, so we've only seen her at a distance (through an open window!) as she couldn't bubble with us as it would have been too risky for me.
We've all had both vaccines now (except the kids), so we're now meeting up outside (still socially distancing) and we're going for lunch together (outside) tomorrow, which I'm really looking forward to!
In more "normal" times, I would see her at least 3 times a week and she would usually meet up with our DDs too, perhaps pick them up from school and have them round for tea, or take them swimming on the weekend. We talk on the phone every day though, sometimes more than once. Because she lives on her own and has chosen to live near us, I do feel some responsibility to see her regularly and do find myself inviting her over for dinner even when we could do with a quiet evening - I would hate for her to feel lonely.
My DSis lives 300 miles away and she and our DM have a difficult relationship, so hey only see each other 2-3 times a year and speak on the phone weekly. DSis takes more responsibility with our DDad though; sees him more frequently than I do and we both speak to him weekly.
I don't think YABU to want the odd weekend to yourselves, particularly if you give your DM plenty of notice so that she has the opportunity to make alternative plans. Perhaps you could liaise with your DSis to work out some sort of schedule to ensure she doesn't have the option to get "huffy" with you. In the grand scheme of things though, I'd rather have my DM over to ours for an evening when it's not really convenient than for her to be at home, feeling lonely. Her own parents both died at only a few years older than she is now and in spite of everything, she's my best friend and although it doesn't necessarily follow that she will also die relatively young, it does make me think - and I know I will miss her hugely when she's gone.