How to handle PTSD?

So I'm 14 and I was diagnosed with PTSD, and also a Mood Disorder. I'll explain my situation , when i was 8 i found out I had a half-brother.. his mom had died and i only found out about him a week before he moved in with me(he was 14 when he moved in). A day after he moved in with me he came home drunk and started beating my mom, my dad tried to keep him away from her and my mom called the cops crying and an ambulance and cops came and i was crying . My brother later was diagnosed with bipolar, he was a druggie and more . Between now and what i'm about to mention the cops have been to my house about 8 times or my brother was taken into mental facilities. This year in January he punched out 2 windows in my house and threatened to stab my dad, my mom called the cops and I was with my boyfriend.. I was upset and my boyfriend took me out and walked with me around my neighborhood until the cops left with my brother. The next day we got a restraining order on my brother, it is now April and i'll be in school (i'm an honors student) and i'll have random breakdowns and start crying and seeing everything as if it's happening in front of me like flashbacks.. and i don't mean just that day every single time. As well as those, when I was 6 i saw my Grandfather lay in a bed in a coma and he sprung up spitting out blood the day before he died, and i have had nightmares about it since, i see it over and over again when i sleep. I haven't gone a day without crying for a long time.. how do i handle the flashbacks and whatnot ?