I noticed this with my parents when their world "shrank" for want of a better word.
Firstly during covid as my father was shielding, then after as they had already got used to not going anywhere or seeing anyone and it got worse last year when my mothers health issues led to her having to stop driving (father already couldnt drive due to sight loss).
They have nothing to compare things to. So when I dont answer straight away, even though I put my shifts of their calendar, I get a panicked voicemail asking if me and the kids are ok. Its all they have, its all they think about. They have no frame of reference that tells them "Its only been half an hour!"
I get it, it comes from love, but FUCK ME its annoying! I say to my boss "I am sorry, I have to call my parents", she understands I am their main carer, and it turns out that its because the water pressure is a bit low and they need to know if mine is the same but because I didnt answer, me and the kids have probably been slaughtered by an axe murderer.
I think your MIL has very little apart from FIL to focus on now, and talking to you probably feels like her only contact with the outside world. I get it, I really do. But I think patience and understanding is the best way to handle this.