That would be very tough for you, but I would be leaning towards thinking that maybe your niece is having problems regarding mental health issues. I wouldn't be at all surprised if my mum said this about my own 34 year old sister. Outside everything can look bright and fun, but she could be battling demons you don't know about.

If I were you, I would make sure that you got time alone with your sister, even if you have to put pretty blunt and say "your mum and I need to chat alone, so we are going to have a coffee in this room by ourselves for a while".