Is this a weird thing to do?

ever since i met my friends (more like aquaintance now...) brother *who is now my boyfriend*, i have just... liked him so much! I may aswell say love! he is such a nice guy; he respects me and for the 11 months weve been together he has never made me do anything i dont want to. he is very caring and he is so fun to be around, he is the nicest guy i have ever met. but the problem is that his sister has been spreading rumours about him and i, like me being pregnant to him etc etc... it really hurts my feelings and ive told him this and me and her have also fought about it before. i am just realising that i have been so stupid to forgive her as this isnt how friends should be towards each other. i dont consider her as a "true" friend because of this, is this wrong?? and i am thinking of changing schools because i am constantly being harrassed about whether im going out with him still or not?? i really love him and right now i have no intentions of breaking it off with him as i have never met anyone like him. im 15.. i know its probably going to sound stupid after reading all that but its how i feel. but my question is: is it weird that i am going out with my friends brother?? the only reason i dont know is because im not sure whether she is a friend of mine... to me shes more of just someone i talk to every now and then... but i was friends with her before this guy happened to me its just that weve drifted apart?? please help me :(