My FIL ruled the house when he was alive. MIL has been diagnosed with mild dementia but we're having issues with quite a few things regaring hygiene and behavior. She is sweet as can be with my husband and son but downright mean to myself and my DIL. Everyone says it's the dementia yet in the middle of a rant, if my husband walks in she immediately turns into a smiling sweet lady. He didn't quite believe me when I described our conversations until he came up behind her one time and listened for about 10 minutes. If she can shut it off that quickly then I don't think it's dementia. Any advice?
What is the benefit to your son and dil of living in the house with mil? (If I remember correctly, she has an attached apartment and refuses to let anyone in.)
If it's not his own blood relative, not sure why there would be any expectation for him and his wife to help the woman at all. And if she's mean to them, all the more reason to stay far away (actually, don't they want to move out?).
What does you husband think should be done? Another thing coming in here is she is treating her son and grandson well, but the in-laws who are their wives poorly. She may be suffering from Matriarch syndrome and think the in-laws are outsiders.
But a) you can't really tell which is Vascular Dementia (especially loss of inhibition, as the dementia affects the temporal lobe) and which is competitive misogyny, as is/was so common in your MIL's generation; and b) even if you could say for certain at this point, the same balance won't necessarily be true next week.
So speaking for myself, I'm not sure it helps to try to work it out. And I certainly would not waste a microsecond trying to correct your MIL's behaviour, whether it's down to the disease or to her unfortunate personality. Save your mental energy for setting boundaries and devising responses to her that you are comfortable with - because that bit you can control.
The thing is, Dad will say things like "I can't help it. You're just too..." You get the idea. He has a genuine, dementia-caused compulsion. He really can't help it. Yet he somehow manages to not stare at me, or make suggestive lewd comments when other people are around. So he has some control, right? Or maybe not. Something is set up in his demented brain that says wait 'til she's alone." But he can't exercise that control when what he sees as opportunity presents itself.
This sort of thing may be what you're seeing with your MIL. The possibilities of what triggers her behavior are endless, but she may just be compelled to what she does and have no empathy at all for those she does it to. (My FIL fails to see how his behavior could make me uncomfortable or angry, or that it would affect his son if I did, for whatever bizarre reason, give in to him.) Just throwing this out for you to consider.
My husband, and extended caregiving family, have gone round and round about whether Dad's behavior is dementia. If he doesn't know it's wrong, why only in certain situations? Why be so sneaky? Can people with dementia be sneaky? Yes. Yes a hundred times. While reading other people's experiences on this board, I've learned dementia doesn't take away the sneaky. Parts of the brain work, other parts don't. The sneaky part seems to persist somehow and the dementia patient may not even know they're doing it. It can get a little surreal. I hope you get more answers and maybe some good advice on how to handle your situation. In my case, I've had to learn how to simply put up with a lot and avoid being alone with my FIL.