Although it started out as play fighting it looks like he's learned something he can do to get his "big brother" to do what he wants, he feels like he has power and is stronger than you, your Achilles Heel if you like. And the more you do for him the more it's going to reinforce it. My advice would be to put a stop to it, however necessary.
Your parents probably don't believe it's not serious and just see it as "boys will be boys", or that it's all exaggerated. Your father sounds similar to mine in the way that he expects you to get yourself out of situations like this.
You could try poking him in the side (between the ribs and pelvis) which will probably cause him to let go (Your hand is right there in the pic). I don't know if you've ever had it done to you but it's a weird sensation, between a tickle and pain. The harder you prod the more it becomes really uncomfortable. Or pinching a sensitive part of his body, like his toes or skin on his legs. But don't be afraid to hurt him because not only is it self defence but he's more likely to remember.
Your parents probably don't believe it's not serious and just see it as "boys will be boys", or that it's all exaggerated. Your father sounds similar to mine in the way that he expects you to get yourself out of situations like this.
You could try poking him in the side (between the ribs and pelvis) which will probably cause him to let go (Your hand is right there in the pic). I don't know if you've ever had it done to you but it's a weird sensation, between a tickle and pain. The harder you prod the more it becomes really uncomfortable. Or pinching a sensitive part of his body, like his toes or skin on his legs. But don't be afraid to hurt him because not only is it self defence but he's more likely to remember.