I agree with downwiththissortofthing.
Make very sure your kids aren't bullying or abusing each other.
Set a family meeting and ask them to contribute ideas toward making home life safe for everyone. Construct a contract, and you all sign it.
I'm my home:
I am safe
I am loved
I am relaxed
I am supported
No one shouts
No one hits
No one bites
No one sneers
That kind of thing.
I was beaten up regularly by my older sister, and once knocked out. She had MH problems when she was an adult, so you may need to have your children assessed for MH problems.
A friend of mine was raped repeatedly by her older brother, her mother couldn't work out why they didn't get on, and used to lump them together more because she thought this would fix things.
This kind of violence and abuse isn't normal.
They may need separate schools, friends and hobbies. You will need to facilitate that.
Don't lump them together unsupervised or let one spoil something for the other with mocking and sneering.
You need to keep up this work until all bad behaviour is gone.
You need to give each one, one on one time with you.
Exhausting, but be careful there isn't bullying and abuse going on.
What kind of things are they seeing on YouTube?
Do they eat wheat and dairy and sugar. Try a vegetarian whole foods diet for a while.
I think you need outside help actually. Sounds like the dynamics are very broken. This violence isn't normal horse play.
IMO You need to take immediate action. Punching to the chest could result in death. This is serious assault.