unfortunately some older people are bitter and negative
maybe so, and sorry to hear you're having a rough time at the hands of your mother in law but you're letting the same negativity and bitterness manifest itself in you too .
I wonder if she would treat me the same if I was English-born
If you were English born you would have known to stand up to her immediately instead of letting it drag on for 12 years and i'm sure your mother-in-law would have respected you more for it.
Only solution is to start standing up to her. And if you can't, it's only a matter of time before your english-born children sees what's happening and starts standing up to her.
As they say, what goes round always comes around. Good luck.
but please, in future, don't generalise and say negative things about people, especially your host's culture. It just doesn't go down well.