Older boys picking on my youngest

I have 4 boys. Fraternal twins who are 16 my oldest who's 18 and the baby of the family, my almost 15 year old. My older boys constantly pick on my youngest. They take his things, hit him, embarrass him in front of his crush. They scared him really bad by pretending we were being ribbed causing him to wet himself. And they made fun of him about that for weeks. He has really low self esteem. I tell him this is just the price you pay being the younger brother. Sadly with being the oldest the biggest privilege is tormenting the youngest. My youngest also has a different father who is in prison for 50 years for vehicular manslaughter. That whole situation has stressed him out and caused him to have accidents at least 3 times a week and he shares a room with the twins and they loovvveee to torment him over that. I really want to build my boys self esteem up. Any ideas how I can help him with his self esteem?

Edit: My older sons dad.... Long story short he got deported back to Russia. I don't want to talk about how it happened... But that's what happened. He talks to our boys via Skype.

Edit: Idk how my oldest would feel about sharing a room while his younger brother gets his own room. That see!s unfair since another privilege of being the oldest is you getting your own room.

Edit: Idk why people always resort to kicking their 18 year olds out. Also don't know how it is in your state but in mine as long as they are still in high school you cannot legally kick them out until they're not in High school anymore or 20. Whichever comes first.

@Lisa I'm not scared of my kids. I just understand that's how teens are. I still love my boys and sure their father in cam sooo much from Russia😒. And the purpose of the law is to help allow kids to still finish high schools because parents be ready to pick their kids out the second they turn 18 for no other reason than just being 18. Attending High school homeless is hard