My mom is planning on taking her mom into her house to live after we leave as live-in caregivers. There is not really any family left that is willing or able to handle being my grandma's caregiver. After being her caregiver for about a year now I do not think it is a good idea for her to stay with my mom. It will take over her life and I don't think it's the best option for my grandma anyways. She needs interaction during the day, more than my mom will have time for on the days she is not at the memory care center. I would hope that my siblings who are still living at home would help out but realistically I know they have jobs and school that take up too much of their time.
What other options are there? Just a nursing home?
There are lots of factors to be considered, but I agree with your initial reaction. For your mother to take your grandmother into her home should be a last resort. The arrangement will become increasingly stressful and difficult for your mother while your grandmother will become increasingly comfortable with the arrangement and dependent on keeping the status quo. It will be much easier to arrange care outside your mother's home now than to move your grandmother out of your mother's home a year or two from now when your grandmother has become settled there. Please investigate all the options and engage a geriatric care manager or consultant if needed. It could be well worth the small initial cost.
In-home care is another option. If your mom brings your grandma into her home your mom can hire caregivers to supplement her caregiving duties. She can have a caregiver just to do bathing and grooming (several times a week) all the way up to full-time depending on your grandma's needs and your mom's needs.
You speak from experience when you say caregiving will take over your mom's life. It will. We all know that. If your mom finally decides to take her mother into her home be as supportive as you can be. Be there for your mom. I get the impression you live out of state but phone calls and cards are always nice and are a good way to let someone know you're there for them. Your mom is lucky she has such a concerned daughter.