Since January this year ive been getting very close to a really nice boy. I bet you're thinking 'she fancies him' or w.e. But actually i don't, he sees me as his best friend and i feel the same.
In January id had a stressful time with an ex boyfriend and i was often depressed and crying. He helped me through and a few weeks later he split up with his girlfriend of 13 months (or so). I also helped him reassuring him that everything was going to be ok. And it was.
For weeks after that he was so nice to me. Helping me and supporting me no matter what. Its just latley he's been quite nasty. And he only ever seems to be nice when he wants something.
But there are those occasions where it seems that he is being nice because he wants to and not because he wants something. I think he's starting to not want me around. Im going out with his stepbrother and he doesn't seem to like that. I wish i could explain how i feel. But i dont even know what ive done wrong. :/

He likes you for more than friends, and you are making him upset and jealous by going out with his stepbrother. Odds are his stepbrother is probably adding to his jealousy too. If I were you, I would get out of the relationship and break ties with that family. You should not come between brothers ever, and you shouldn't date your friends family members. Usually you end up losing both a friend and a relationship in the process.