You and your sister might consider buying her home from her and giving her a life estate. You can make the terms of the life estate as best suits the circumstances(any conditions that apply). You should talk to a real estate attorney for advice on what conditions are allowed and you want. If you are unfamiliar with the term, it means you own it and she gets the conditional use of it according to the terms applied. It may help her get some government assistance she would not qualify for as a homeowner.
Moms and Dads are very special, take care of them as best you can.
Good Luck.

 

Talk it out with your sister and your girlfriend and make a plan. Your mom's not going to get any younger either I'd plan for the long term so you don't have to have to have more money fights with your sister 5-20 years down the road.

I can understand how your girlfriend feels. Your mom is 57, makes you 40 at the most? When you have a few kids of your own she might have a change of heart about your mom staying with you. But until she has that security, she's going to have a hard time sharing you with anyone.

How long has your mom got anyway? How many years do you expect your mom will be able to care for herself before she needs a nursing home or other care?

If she can manage herself for another 10-20 years, why not hang on to her house for a while. If you pay her mortgage, it'll be yours when your mom leaves it to you. Or you can use the equity if she ever needs to move into a nursing home and you won't have to have this discussion all over again with your sister. Or support mom until she qualifies for that reverse-mortgage and can live on that.

Renting out part of her house isn't a bad idea either if that ranch house is big enough. Renting out the basement isn't worth it if you ask me. My friend gets 300$ for his basement. It's a nice basement, he could get $350. But it's still a basement. He could get $1000/month for the upstairs. And if he did that he could live downstairs, and use the rent from upstairs to pay the mortgage.

Why not put mom in a little apartment in her own home, give her the privacy and everything else she needs, not too much housework, you keep access to the downstairs for maintenance. And you can rent out the upstairs for a lot more money than you'd ever get for the basement. And people who rent a house for $1000+ they just want a nice house to live in. They're not going to make a mess or be late with the money like poor people who live in basements.