The hard part is over.
I'm matching you with one of our specialists who will be calling you in the next few minutes.
With advanced dementia, if Mom is told, she will grieve... then an hour later she would forget she was told. She would grieve each time told.
Thus any time Mom asks about your sister [sounds like she might not] just say she is busy and will visit next week.
Things have a timing and if your Mom is stable, doing well, relaxed & settled down, then why rock the boat and tell her. If she starts to go crazy in a few months and keep asking to see her everyday, then you might have to tell her.