My cousin is blackmailing and threatening me and my little sister?

A person can only blackmail you if YOU allow them to. Your grades are slowly dropping because you are ALLOWING them to. Your sister's self esteem is being lowered because she is ALLOWING it to be lowered. If you didn't ALLOW your cousins to blackmail you, you wouldn't have the problems. Sounds to me as if your cousins are teasing you because they know they can push your ons by doing things this way. They are having a great time at your expense and you are ALLOWING them to. When you stop making a major issue out of all of this they will discontinue their threats. But right now you're making life fun for them.