Old person here ( though not as old as your MIL). I think I can explain the secondhand mugs.
as people get into their ( what they think of as ) last years, they don’t always want to spend money on ‘expensive’ new stuff. They think ‘oh, this will do’ ( my mother used to say ‘ it will see me out’) . We come from a period when people loved a bargain! Plus everyone tends to think that stuff has got much more expensive compared to when they were younger, and unless Australia has escaped current inflation, there’s a grain of truth in that.
other posters have talked about the filter declining, that has been a useful observation for me, I can see that happening with my DH and probably myself. I think that may help to explain the mug story as well, it’s a train of consciousness thing, she thinks ‘o mug, mugs, I just bought some mugs…..’ it’s not a comparison. It may also be that she doesn’t get much conversation at home, so she just wants to share her life a bit.
If your FIL is becoming a ‘hermit’ , I wonder whether there are other MH issues there, and your MIL , that is your husband’s mother, may be having quite a difficult time at home. That can make anyone rather less sensitive to other people when they finally escape the oppressive atmosphere. Any chance that your DH could explore some help?
please don’t think I am victim blaming here. You have said that overall she contributes more to your family’s well being than not, so it is worth trying to stop an escalation of bad feeling.