When I found myself divorced after many years with no family in the country, and severely depressed, weekends were a huge struggle for me; with Sundays being the hardest day of the week. As that is the day, people usually spend with their families.
I hated feeling like a burden to others, but you can't just turn off your feelings, and being rejected when you’re grieving is hurtful.
It may be an inconvenience for you, but it's a huge deal for her. I think you should show her some grace, but equally your husband should
Do more for HIS mum.
Are Sundays very formal? Could you change things up? Have her go out with DH and DC. Or even have an easy meal and sit and watch a couple of films together. Maybe go for a swim, get her to help chop salad…? If you have a spare room, she could stay over that day or from Friday, so it feels less like you're entertaining, and more like she's part of the family and just hanging out together. That way you can spend time in your room reading or watching telly, or going out. There's nothing wrong with asking for some down time too. (I have to do this & if everyone knows what you need this & you're upfront then it's fine).
Even if you're not attached at the hip all day or weekend, it's nice to know she could come back to a comforting environment. I reckon you could even encourage her to make plans, and do a bit more.
Having company is lovely, and makes a world of difference when you feel like your whole world has fallen apart, and the feeling of loneliness and the grief is so loud.