Back ground first my sister is bi polar for the last 3 years she has been very stable after a decade of episodes and misery for us all. Dsis is isolating with DN(12) they have been doing well however my sister feels that she's struggling with her moods more and more. DN is fairly typical for her age dramatic and lots of energy she thrives in school and stays with us a couple of days a week usually to allow dsis some time on her own. It's been this way since she was little she's never spent 3 full weeks with her mum in her life.
I can see the dsis is terrified of having an episode and is feeling overwhelmed being with dn 24/7 (I know it's her child but her mental health has never been good enough to parent alone she needs support) I want to bring DN here to allow dsis some time to get even again but I obviously don't want to refuse to let her go back after a week or 2 if that's what they both want.
We have been dealing with this long enough to know the signs and thankfully dsis is now in a good enough place that she knows them, usually if she felt like this DN would come to us for a while but with the isolation I'm not sure what's for the best. Until 3 years ago DN didn't live with dsis due to periods if neglect nothing very terrible she was fed and watered but that was about it which is obviously bad enough. I don't want to leave it until such time where dsis is mid episode and DN is stuck there with her or undo the 5 years of work dsis has done to get where she is but I also don't want to break the rules.
So YABU - dn needs to stay where she is unless she's in danger of neglect.
YANBU - in these circumstances MH takes priority bring DN to stay with you