My boyfriend wants to move his mom (84) into the house we have just purchased, without my consent. I don’t want to be her caretaker. Advice?

My boyfriend and I are closing on a house next week, we’ve been renting for the past year. He just found out that his mom has squandered away over 100k in the past 4-5 years from her reverse mortgage and is now worried about money to support herself. He said if she needed to move in with us she would, didn’t ask, just said if she needed to she would. She is 84 and stubbornly unsafe and a huge fall risk with everything she does. I do not want or feel responsible for taking care of her as she gets further along and more disabled. I have met her twice and have no connection to her and I’m not even married to her son yet. I have a back injury that won’t even make it possible to help her with the slightest of transfer tasks. My boyfriend works 10-11 hour shifts and says he’ll take care of her and I’ve told him he has no idea what 24/7 care for an adult who out weighs him or I is like. I took care of my father who had a severe stroke almost ten years ago and it was mentally the hardest thing I have ever done and my thyroid basically died from all the stress involved but I loved him to death and I’d kill myself to do it again if he were still alive. I also don’t want her to pass away in my house as he thinks is only a matter of time. I don’t want to find her dead one day or have my son find her. I don’t want her dying in this house we are starting our new life in after my divorce from a 20 year marriage. I don’t love or even respect my boyfriends mother, she is the reason why my boyfriend is so lazy and disrespectful at times and I shouldn’t have to pay 35k of my own money for a down payment on a house we share ownership of (he isn’t putting anything down, but is going to make the mortgage payments once we are in) to be forced to care for his mother and support her living with us for what could be 10 more years. He has no idea what could happen and how much care she will need. Her living with us would be like having a permanent house guest that is a stranger to me. We don’t have room for her and she will have to live downstairs and come upstairs for everything she needs. I swear if my boyfriend says we have to give her our master bedroom upstairs because she “deserves the best room in the house” I think I may explode. He lets her have the best room anywhere they go on vacation and would make me sleep on the floor to give her the king bed somewhere instead of letting her sleep in the twin bed so everyone has a bed so I wouldn’t put that past him. He can’t afford to pay to have someone take care of her 24/7, and I know he will eventually use that as an excuse for why I have to take care of her since I dont work full time (which the part time job I have may not last long due to my back injuries). I paid extra to have the third bedroom for my 20yo daughter, who may return home one day and I’ll need to help her (my responsibility) to get back on her feet. His mom could sell her house and live on the 100k proffit for several years without coming straight here, but he’s telling her things like “we’d love to have you here, we’ll have a blast”, not “well if we need to have you live here because of financial issues, you’re more than welcome”….I have told him that her living downstairs and sharing a bathroom with my son and not having anything she owns here and no family or friends and having lived her whole life in one town is ridiculous, she won’t be happy. She won’t see her son more than an hour a day and I am not going to become her sole caregiver and make myself Ill and further injure myself. My daughter won’t even be able to visit if she lives here because there would be nowhere for her to stay. We aren't even married and I think every possible alternative should be exhausted before even mentioning that she move in, but he’s acting like it’s happening and he’s getting her all hyped up thinking she’s coming here to live with us.