He is just being irresponsible and unkind. At his age, he is supposed to protect you, not victimise you. But take heart. You just need to calmly make him know how you feel.
Try writing him a kind email for him to consciously re-visit his mistakes but when you do this, avoid tough (strong) nor hateful words. Write in question format like you are wondering, but not blaming. (an example is: I don't like running to mum and dad for every wrong you intentionally or un intentionally do at home, but "How does it make you feel like when you mess up the house, use my stuff without my permission and not getting a complaint from me? Would you feel nice if I took some of your stuff that I like? Tell me, do you think You and I would be happy if I found some boys bullying you and just cheer on? etc)
To secure yourself, make a Bcc of the mail to your mum and step dad or a trusted member of your family, who will step in the gap for you, in case some members of your family gang up on you or that boy scape-goating his actions over anything against you.