My friends think my best friend fancys me

Me and my best friend always talk on the phone about weird shit. She doesnt seem to happy when I go out with my other friends. Sometimes she will cause an arguement when i go out with other friends and not speakto me but its not long before she apologizes for her overeaction and we are normal again. She will pick arguments all the time over little things and she is never happy about ny boyfriends. When we are out she cock blocks a bit. I call that being protective but my friends say she fancys me because she also hates evert person ive ever gone out with. She tells me its because shes sees me as a sister and doesn't want to know however she tells me every thing about the boys she speaks to. She can be a little nasty to ny other friends but i always back her up as she has a small case of bipolar which i believe could be the trigger.
She takes so much care of me and looks out for me more than any friend would. She calls me hr best friend but all i do is make her laugh, she makes ne feel so special about myself however when shes horrible it kills me inside i dont know if this is love for a friend or some thing more