My parents might be getting divorced, how can I be sure?

My parents have been together for 15 years now, and alot of stuff has happened within my family since then, my mum used to smoke, but gave up before i was born (i'm almost 16) and recently, we have been going through alot of stress, We recently found out my older brother (17) has been a drug dealer and user for about a year now, and my mum has been going through grief as we live on an estate, there is alot of gang violence, and she was left traumatised, she is on anti-depressants and has also picked up smoking and drinking (bottles of wine), my father had liver failure about 8 years ago, and very nearly died, and my nan (dad's Mum) died 5 years ago from smoking and my dad's brother is a heroin addict, my father hates them all, and he hates everything my mother has become, i even woke up to a letter asking my mum if she wants a divorce, they have been fighting constantly, and i, myself, have been suffering with depression for a while, no clinically, as only my two best friends know about it, i also used to self harm, and from time to time, i do, i also went through a stage of being addicted to pills, just any pills, pill tails, OD sometimes, but i have always survived, i have been suicidal since i was in year 6 (aged 10) and no one knows my whole true story, but if my parents split up, i will have to most likely move to conrwall/devon with my mum, and considering i live in London, i don't want to go, and never see any of my friends and part of my family ever again, it will most likely be the end of me, what do i do?