This is just my take on your problem.
I realise you and your wife are often apart for reasons of looking after elderly parents in Canada and America- so from a distance you aren't really going to improve the situation. The things that improve the situation rely on a consistent and predictable attitude/approach by the parent(s) which you cannot offer (even if the mum wanted that) - because you just aren't there full time.
All the conversations about the things that she finds difficult will have to be had by your wife as she is the full time person on the scene. The fact that you also find this difficult, I think is a side issue as really you are still there as a "visitor" at the moment.
If she has tolerated 6 hours of TV or eating and leaving food about the house or clothing scattered etc then that is the normal thing in this family and without mum being focused on change and enlisting the help of her daughters it just won't happen.
If she is repeatedly complaining to you maybe she has got to the point where she wants it to change.
If I were her I'd probably want to sit down with my daughters and explain why I need to change and ask them (as young adults) how they think i could achieve that change. I'd point out the benefits of the change and would hope they would want to help. In my experience even grumpy teens will try to help if they can see a clear way to do so.
It is well documented that we all like our own ideas best. They need to develop ideas about change for themselves and also see pay offs.
When I wazlooking for help at home I'd often explain how I thought what I wanted might be useful to others (I.e. them)
I don't think this driver of change is really a role for you - but if you see even small positives you can comment on them (without being patronising).
You need to see how committed to change your wife is first I think.
Good luck.
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