Suicide

Hey! I need advice!

So my sister-in-law’s little sister, let’s call her M. M is about 12, and for the past 2 years has been going through weird “phases”. M has talked about her being depressed, so her mom took her to therapy. M talked about suicide, so they took her to this thing (forgot what they call it). She has been there numerous times. Recently, she was taken by cops, I believe they call it 51/50. Now today, she has been making false claims about her mom abusing her, and she now has CPS involved. She also made threats to her school and might get arrested. These behaviors have been happening for the past 2ish years, and they have all been addressed. I should add that she has been a pathological liar about certain events happening.

Does anyone know how to handle situations like these? My sister-in-law is struggling through this, trying to help her mom, trying to help her sister M. But there’s not much left. Any tips??