My younger sister is 18, but still in high school and lives with my mother. She'll be normal for months but then, seemingly out of nowhere, she'll start abusing my family (mostly my mother) by screaming and cursing at her, threatening to kill herself and writing a suicide note saying she did it to punish my mother and post it online, punching herself and pulling her hair, threatening to move out, etc. This is the kind of stuff you would expect people who are actually abused to do, but I know for a fact that she isn't abused by my parents because I lived with her for most of my life and she would do the same shit to me when she didn't get her way with something stupid like I wouldn't drive her to her friend's house. She's obviously has mental issues but we don't even know what to do at this point.
Preface not a professional or aught, but look up borderline personality disorder. I've done quite a bit of reading on it recently. CW on the things that might accompany that: Quick synopsis would be: it very often accompanies child abuse but not always, it can still occur in loving and non-threatening households, it's marked primarily by an intense and unabating emotional dysregulation - basically too much feeling (sometimes good, mostly bad) in the wrong proportion. Self-harm and suicidal ideation and successful completion of suicide are marked. People with BPD are often thought (possibly harmfully) by the public at large to be emotionally manipulative. BPD sufferers can enact something termed "splitting", moving from complete idealization to complete devaluation of another. Significant increases in aggression or impulsiveness. Marked fear of rejection or abandonment, etc. etc.
But regardless of whatever it is that is absolutely something that needs to be treated by mental health professionals, ideally the sooner the better, because whatever the cause is, those behaviors are severe and point to something significant.