And, with my healthcare hat on, GET IN HELP. Virtually any functional adult has the basic capacity to provide basic care for your father. Only your mother can be a wife to him. Only you can be a daughter. Outsource, whatever you can and conserve your capacity to do what no one else can. Don’t waste it on doing the washing and making sandwiches.
There is so much pressure on women to ‘care’ and so much conflation of ‘care’ with ‘domestic work’. They are not the same thing. (I don’t even like calling it care. Love is not measured in hygiene, nutrition, clothing or medical assistance. And hygiene etc can be maintained very impersonally and uncaringly by a loved one or very caringly by a stranger. It can even be used to taunt. Impersonal is better than that!) You may want to consider keeping your hours and buying in help.
Your father is, sadly, only going to get worse. In unpredictable ways and at an unpredictable rate. Getting in help before he desperately needs it builds familiarity and capacity so that new people become familiar to him and skilled at caring for him in the ways which matter to him while he can still advocate for his preferences. That’s very, very valuable. It will help you and your mother feel less alone with it, and possibly bring a little objectivity to an emotional situation. It could be a long road and you don’t know how long your reserves will have to hold out for. Look for help to come in, but also groups and activities to go out for. (I’d be beside myself with no time alone in my house if I had caring responsibilities. And also if I couldn’t go out without or have time apart from someone who obviously resented caring for me, even if I understood I was unintentionally hard work and knew they loved me)
There are also genuine concerns‘s about your dad’s well-being, you need to act on those. It sounds to me like your mother is acting abusively. While that is absolutely understandable, because dealing with. dementia can be unbelievably taxing and frustrating, that doesn’t excuse it. Your father has the right to freedom from abuse. He is the most vulnerable person in this dynamic, needs the most protection and his needs come first. That does not mean I discount your mother’s needs, just that he is more vulnerable than she is and needs your advocacy and protection more. I would genuinely consider some sort of virtual monitoring. I would try and get it in tactfully, or via a third party, in deference to the difficulty of your mother’s situation, but I would also make sure she knew it was there as a deterrent and an incentive to manage her own behaviour.* It’s astonishing how the frequency of abuse declines in line with the likelihood of being Witnessed. It will improve your father’s wellbeing not to be subject to verbal abuse And to feel safe, and I imagine your mother doesn’t feel great about it. It will prevent escalation and give you peace of mind. I don’t know what’s out there these days, but technology for his benefit, like wandering monitoring, could definitely overlap into quality of care monitoring. Even if it’s just something that can be activated remotely and you don’t. In tandem with this (because it IS incredibly difficult) I would look at support for you mother. Therapy, groups, breaks, peer support etc. it’s HARD for women of that generation, who often had the dirty end of the stick with both being expected to work outside the home AND not being able to expect much help in it. She likely resents your father for exploiting her, and he likely did and doesn’t acknowledge it because it was normal and doesn’t see what he can do about it now. That doesn’t make it any easier!)
*My own mother behaved abusively when she believed she was unwitnessed. Never in front of anyone else. She claimed this was excusable and unavoidable, because of the intensity of her feelings. Since I let her know I have cameras running 24/7 there has been no abusive behaviour. A truly remarkable coincidence. I‘m so surprised.