I think there are possible nuances here - during 2020 I suddenly got a lot more interested in wanting daily check-ins from loved ones who lived alone! If Mom lives alone and is retired/works from home, it might be reasonable to want to know you have a daily check-in with someone who would be able to call in a welfare check if you suddenly stopped, and if Daughter is also living alone and may not have a lot of connections in the country, asking for one thumbs-up a day as a check-in probably also isn't unreasonable. (Covid isn't over! And there are lots of other reasons that might be useful!)

But to the extent any of this is LW's business at all: if they are in situations where a daily check-in might be reasonable emergency preparedness, mother and daughter don't have to be each other's contact. LW should volunteer to do a daily check-in with his sister, if they don't already, and reassure her that he can be emergency contact with daughter if something happens, and that makes more sense anyway with her out of the country. He can also ask Daughter to give him the name of a contact like that in her country, so that Mom knows there's somebody who will be able to contact him in an emergency too.

And then he can side with daughter guilt-free every time it comes up after that.