A place for mom to live. What should we do?

Kind of interesting how this was written - the second sentence seemed give us two reasons for not taking mom in, but in all reality, the reason is neither you or sister want her to live with you. One bedroom apt is more to avoid just out and out saying it. Didn't sugar coat it like that for brother. Just put it out there he doesn't want her. -- Just happen to notice that.

Mom's kids do not want to share their homes with her, so what are you to do? You know the answer to that. She needs to go to memory care or assisted living so that someone is watching over her. Don't wait for the big accident.

You say she doesn't want to spend the money, indicating that she has the money and can afford this type of facility. Find an assisted living that has a lot of activities. If she has any friends that are in one, talk to their families. Make sure you have her in a facility nearest to the one kid who is more likely to go visit and be an observer for mom. If she mentions the money, tell her that's why she has it- to take care of her, keep her safe, and not having to be alone. Be upfront and honest that none of you can take care of her. just omit the one bedroom apt, no room thing because if she has money for AL facility, she definitely has money to upgrade the square feet of your apartment. When she offers to do that, then you gotta come up with excuse number two. So don't go down that road.

With the dementia she really can not make decisions about her care, her safety.
You can look for smaller group homes that she might like better.
But Memory Care is what she needs or will need in the very near future.
And with the LBD please know that that type of dementia often comes with violence. Are you prepared to handle that?
I think if you tell mom that in order for her to live with you or sister she would have to pay the extra cost of a larger apartment. Also the increased cost of all the utilities that come with it. That might actually equal the AL cost. With AL you are not paying for insurance, utilities, food (in most cases) and transportation since most offer transportation to shopping and doctor visits.
In any case mom is quickly loosing the capacity to make decisions the decision to move her to Memory Care is going to be yours and your siblings.

You find a Memory Care Assisted Living residence for your mother since a person with LBD dementia should NOT be living alone! Most people are not qualified to care for a person with this type of dementia at home, so let your mom know that her needs are more important right now than her wants; her safety must prevail! If she does not have the funds for private pay Memory Care, then you'll have to apply for Medicaid to have her placed in Skilled Nursing.

Best of luck!

I guess she goes to memory care (not assisted living).

No one really wants to leave their comfortable surroundings for the unknown, but Lewy isn't going to get better -- it'll get much, much worse. Neither you nor your siblings will be able to handle it after a short time, so she needs to understand that for HER health and safety, moving someplace where there is staff trained to help her (plus plenty of social things to do and people to know) is the only viable option.