Is your mom right if you like your stepbrother's friend because he treats you like an adult but your mom says it's just a school girl crush?
Many young people can form crushes on different guys (even older men) and particularly when they treat you as their equal. Yes, your nan and mom are correct, but, whether it be a crush or true love, it's important to you and shouldn't be taken lightly. It's OK to have a crush and you may have several before you find that right guy for you. For now, remain friends and perhaps when you are couple of years older your step-brother's friend may just ask you out. Miracles always happen.
I feel like the third wheel between my friend and her best friend when they're together they totally ignore me but at school me and her are like best friends almost?
Is he just a friend? or a boyfriend? If he is just a friend and treats you like a princess maybe he is trying to tell you something. If he is a friend consider him as being more, would you like it would you not? Then once you've made your decision talk to him about it and see if he has feelings for you. If he's a boyfriend then he is trying to be nice…
well if your hoping for romance from this guy and he treats you like a friend, it probably means he just wants to be friends. If your interested in this friend ask him what he thinks about you, then slowly tell him that you think you like him more than a friend. But, warning it might scare him off a little, so think about it.