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.