Hi there, welcome to PC
Is your sister seeing a psychiatrist and/or therapist on a regular basis? Is she on meds for bipolar?
There are many good mood stabilizers that may be of help to her. From my own personal experience, Lamictal was the best for me, for I was DXed as "mild" Bipolar-II, having more of depression than any mania, the mania was mild and may have been triggered from using diet pills (Phenterimine) at the time in late 1999 to Spring 2000. After not using diet pills, I had no
signs of any mania. Sort of think it was the diet pills
that triggered my whole problem.
But that's my own story. Lamictal was good addressing
the depressive end when all the antidepressants (SSRI's) stopped working.
Not knowing your sister's situation other than what you shared with us, there is not much I can suggest except if she isn't being treated by a mental health professional
it may be in her and her child's best interest to seek professional care. Some mother's can be huge triggers, if your sister does get professional care with a pdoc/therapist they may also help her deal with your mom too.
I wish you, your sister, and her child lots of luck with this.

Take care now,
DE