We did this. Sold up, mum sold up, and we bought a larger place together with a ready made granny annex type thing. It was lovely. Mum lived with us (or we with her) until she died, and she died with me dh and dd at her bedside, dd holding her hand. Peaceful and lovely.
I did spend quite a lot of time with my parents after I'd moved out of the family home; my sibings and I went 'home' for Sunday lunch almost every week, so we all knew we got on well enough. That went on for years, really about 30 years, seeing our parents and each other pretty much weekly because we WANTED to.
I can see that you're worried because that is not the habit of SIL, and she and mil have not spend a great deal of time together until this holiday. Sometimes things just happen. Maybe they discovered that they do actually like each other, understand each other, that they gel, etc.
I don't think it's really your business or indeed your dh's, what two adullts choose to do. I can see that you have some valid concerns and I think the advice for your dh to contact both MIL and SiL tot alk it over is good.