My best friend sister likes me i liker her too as a girl friend only.she likes me as a brother..help me please?

when my best friend told me if do i like his sister. i said yes but i was shy,and he told me that im a hard worker and very smart so he was ok with that. im 19 and my best friend sister is 17 years old..when i meet her i knew by that moment she likes me because of her smile and she talks to me alot and always texing me flirting with me every day on the phone or texting. or at work she will come up front where im at all the time working hard. when she is on break. and she will talk to me about school and stuff it went on for weeks and months.and one day i told her that im going to cancun for the whole week.and she was sad.so im starting to haveing feelings for her because she was sad. and i told that ima be about to be gone for the week..and my last day that ima about to leave to cancun. she told me to call her at night so i did. we talked about getting married and having kids i knew she was playing around.so when we hung up the phone and we text about 1 hour at night and we flirt and i told her that i was tired that i need some sleep for tha airport she text :i love you buddy." i was like in my mind she whats to be my girl friend?. i text her back i love u too. when i came back from cancun the first thing when got back i text her that i was here back in houston, we text and then i told her would you be my girl friend. she was like _-------------------------------..i dont know what to say thats all she say and later the next day my best friend text me, that i told his sister to be my girl friend and we talked that was wrong because of that he and he said im her best friend ur my best friend also. and then his sister text me that i was missed up because i ask her out she was sad. and the next day we became cool and.sence that day every thing has change every thing. she wont text me every day like we did. sometimes she will ingnore me often.that feeling is not there no more.but she will smile at me or say hey. why is she being like that? . last week me and flirt alot on break. and this random guy told us we should go out, her face freeze she looked at me. and she looked away and she said i dont date my brother friends .in my mind i was like no no no!. . i dont know what to do? i want things to return back to normal. i lover her as girl friend only. i need help please..should me and my best friend break this friend ship in order to get her? what did i do wrong?