When is a good time to ask mom who and where the siblings you never met are?

I definitely would not expect strangers, or even relatives to step up, now that your Mom is in the shape that she is in, even if she Can remember!

It's time for you to look into the next phase of her living requirements, and not Wait until she is completely out of money, and in crisis mode, to find her safe, comfortable and suitable housing, and I definitely don't recommend that you bring her to live in your home, as it already sounds like you have resentments building, which often happens, when you are unprepared for their continuing decline, I know as this is happening to me, so no judgements from me!

Hopefully you have Financial and Medical POA, so that you can make appropriate decisions on her behalf, so that you can work with your counties Social services, and come up with a viable solution for her care! Good luck!

If this is a family skeleton she has hidden all these years then it is doubtful you are going to get anything from her at this point. I know that my own mom would never reveal her personal secrets when questioned and now that she has dementia she will carry them to the grave.

Mom may not have memory of her siblings. I don't think I would pick this time to make demands about things in the past.

Also, I would not be expecting a person with dementia to be exhibiting grateful and cordial behavior. Caretakers of dementia patient know and accept this. If she needs a higher level of care, then, that's what she needs. I would describe that as moving her to a higher level of care and not "kicked out." Once, the patient is moved to a facility where the higher level of care can be provided, hopefully, it will make a difference in her life for the better. Of course, I would seek the doctor's guidance in making her as comfortable as possible.

Which siblings? Your Mom's siblings, or your siblings?

I noticed on your profile that your Mom has Alzheimer's/Dementia... makes me wonder if she would even know the siblings current whereabouts.