"my dad said it's because i'm a girl which is not a real reason."
You probably don't want to hear this, but of course it's a real reason! I read every day about teen girls or women going missing. I can't remember the last time I read about a boy or man.
With that said, I do think it's a little odd you can't go to sleepovers, but you aren't handling it right. Do not compare with your bro or whine about how unfair it is. I'm sure you did this when you were 10, and it just annoys parents. Instead, have a more mature convo about it where you ask what it will take to make this happen? If your parents don't know theirs, this is a legit concern, because good parents don't want their kids spending the night with strangers. So think about ideas to change that. What if you suggested a barbecue at your place where you'd invite a couple close friends and their parents? You'd offer to help mom organize it. Would that help?
Finally, never say (esp to parents) that you can do what you want at 18. For almost all of us, nothing changes on that day, because we still live at home and rely on our parents for a roof over our head, food and all sorts of things. You get to do what you want when you're fully independent and paying your own way, which I'm will be several years away!