Family Conflict Resolution Steps

I am from Suffolk County New York

My parents don't defend me and Dale, they just let David act the way he does. David is extremely rude and recently asked my mom for my cousin Andrew's phone number, just so he could make a prank phone call to him and David was repeatedly reporting my youtube videos and it didn't even go against their community standards. We don't even talk to David and sometimes we try not to be in the same room as him, but he keeps talking very harsh and saying rude comments by putting me and Dale down we didn't even do anything with him first he just says a lot of painful things about us. When we are in the same room as David we are completely silent, saying nothing, but it is he who is saying things that make fun of us. When I was in a high school program that you can only get a sac with I told my father I cannot get a diploma and he just said make the best of it he didn't care that I might not be able to get a job or go to college even until this day my parents show no concern about my future. My parents don't protect their children, they talk to David's friends who used to come here and physically hurt me and Dale even until this day they talk to a guy named Sean who is David's best friend once when my mom and Sean were in their car coming back from the store sean shouted from "assholes" car at me and Dale as we walked because we were walking around town. They failed to protect two out of three of their children, seeming to favor David rather than us almost always.