M0nica
Was your DH always the favoured son? I ask this, because this sounds to me like a mother emotionally blackmailing her son, with explicitly/implicitly batgaining help now against favour shown in the past.
He needs to explain to his mother that he is now a grown mature man, with domestic responsibilities. Whether you have religious beliefs or not, the bible puts it quite succinctly Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh. He has a respobisbilty to his mother, but his relationship with his wife comes first.
He and she need to realise this. He and his wife need to discuss, not argue, about this. Decide what is possible and then talk to his mother and if she is unreasonable, tell her what you two have decided and if that means accpeting carers or not having somethings done the choice is her.
But I do think more knowledge about her age, and health come into this. For the last 18 months of his father's life, my DH did a 120 mile round trip every weekend he could, in order, more than anything, to help his mother, cope with living with a seriously disabled husband. I usually stayed home with pur young children, and I did not begrudge him one weekend. He was an only child, so had no one to share care with and I knew how ill his DF was. If we had thought this would continue for years, we would have to have rethought things.
I'm presuming, Monica, that you have daughters lined up to look after you when you're elderly and vulnerable — because obviously your sons will be off cleaving to their wives... What sexist nonsense that is.
What domestic responsibilities does the OH's husband have? If husbands are under obligation to be home to do the washing up or the bed-making or lawn-mowing then please let me in on it, because my husband's out most evenings — cycling, going to the gym, meeting up with mates. I'm often out too, doing my own things. OP and her DH don't have young children. He's home at 8pm, presumably with enough time for a chat and a bite to eat before bed. (They sound like Charles and Camilla). This won't go on for ever, things can change very quickly with elderly relatives.