First, I feel the need to warn you that I've worked with a lot of at-risk youth and am seeing this situation through that lens, which means I lean toward early intervention.
Do I need to pressure my parents to pay more attention to it or have them take him to counseling?
Yes, please yes. This is not normal. As you said, he is close to the point where there could be legal implications if he continues.
This is a step by step response to your post, so forgive me if it meanders.
Due to a lack of discipline when he was younger, my brother does not respect authority in any way...
Please consider that this may have very little to do with your parents' lack of disciplining and might have a lot to do with genetics and mental disorders. People tend to think kids are born a blank slate and that everything that happens is because of the events in their lives. This isn't true. People are a product of both nature (genetic tendencies) and nurture (their environment/what happens to them). So in speaking with your parents, don't even go there; they may be completely innocent. After all, it sounds like you turned out really well.
For example, there is a specific disorder called Oppositional Defiant Disorder that might explain the way your brother acts.* There is also the possibility that your brother isn't neurotypical, which can be subtle to diagnose.
I want him to understand that.
It's really hard for most adolescents to think about the future as opposed to now. It's possible to talk until you turn blue without convincing him about that. He has to have a reason to change his behavior beginning now, in the present.
I have brought my concerns about this to my parents many times over the years but I have been told each time that “it is just a phase”...
That says you need a new approach, because they aren't hearing you, or are in denial. If it helps, read more about maladaptive behaviors in children or ODD to gather facts about what is and what is not "normal" teen rebellion. Make a list. Stick to the facts, don't blame, accuse, diagnose or discuss anything but the disturbing pattern of behavior your brother displays and the possible repercussions for him in leading a fulfilling life.
If you are really serious about getting him some help and they continue to disregard your concerns, it's time to set boundaries with them. (Your parents' attention to the matter may not be enough. He may need therapy. The whole family may need therapy.) Read about boundaries (unfortunately most of what you'll find is about parents setting boundaries with teens. Learn what you can.)
I don't know enough about what your situation is to know what kind of boundaries you can set. An obvious one is that you will no longer babysit him until they (parents and brother) see a psychologist. If they say that's impossible for, say, financial reasons, offer to get a job after school and pitch in, or offer to do without everything but food, basic clothing, and a roof over your head. If you use a laptop/ find a way (a job?) to pay your share. I hope you see what I mean. Get around their objections any way you can. Consider talk to other adults who might have some influence with your parents: aunts/uncles/grandparents/pastor/youth counselor/etc.
I wish you the best. You're in a tough situation.
*My older-by-5-years brother was a lot like yours. I, too, thought it was because of poor parenting, and living at home with him was, well, hellish. He basically got kicked out of the house after almost flunking out of high school, and the relief when he left was almost indescribable. He was troubled all his life, and died young. It was only after studying mental illnesses that I realized he had ODD, something that there is treatment for today.
