Hi, and welcome.
I have less advice for the child who brought you here - but it sounds like he got the short end of the genetics stick. Bi-polar and schizophrenia are not usually diagnosed this young - and it's an uphill battle to find psychiatrists who will even consider it - but medication as early as possible is VITAL.
However, I'm concerned about this one: (from your sig)
Child #3 (daughter): 12, Musician, artist, gamer. Angry, scared and confused. Is much like #2 emotionally, hoping she can steer clear of the mess. She tries to help with the other kids when things get tough. Also has ODD, anxiety, depression.
She is young enough to make a difference. And most of this applies to the ones younger than her also, but she is at a critical crossroads. Age 12. Puberty. Heading soon into high school. NOW is when you need to be dealing with the "angry", the "scared", and the "confused". It's way more than grief. You will have no idea what her brother has put her through. Don't bother asking - she won't tell you.
What you CAN do is get her involved in activities that consume her - that feed HER healthy passions. Art and music MIGHT be part of that. Having significant time alone with you engaged in a mutually-enjoyable activity is essential. And yes, I get it... you have a big family, and you can't really take time away from THEM, just for HER. But you need somehow to be doing this for each of the kids - even the most difficult one, if you can. Do you have access to understanding and supportive extended family? They can help look after some of the others while you give attention to one kid at a time - and they can also be taking on their own projects with each of these kids (maybe with the exception of the 16yo...) Somehow, each of the not-yet-in-trouble kids needs to have anchors and healthy pathways and good communication going, in an effort to protect them from damage as much as possible.