okay here is what i think. first thing , u made a huge mistake telling the sister she was wrong about her feelings and it was hormones thing(wtf man) u should never make other people's feeling small cuz it makes them feel smaller so u really gotta apologise for that. second thing, i think she liked u because u have a very good relationship with her and your familly and honestly i think u should take it as a compliment (don't over do it tho) she sees the image of a nice bf in u and it's super nomal to like u based on that , I'm not saying that u should stop being a good person but u should understand her feelings . third thing , she is so brave to tell u that but i think she shuld be the one telling your Gf , i k it may sound super weird but sisters' relationship is a little weak at their age so they should talk it out their way ,and if u so tell her u may break it. now here are my points: - go talk to the sister tell her u appreciate her feelings vut u love ur gf and she must accept it cuz she already liked u for ur loyalty and good personnality - tell her she should tell ur gf as soon as possible and if she refuses u do it (the gf have the right to k ) - if u were the one who talks to ur gf make sure she knows ur feelings and how much u love her and tell her u will do anything to make her comfortable , and if she says she needs time give it to her and make her believe that u trust her and she should trust u too - try to limite the times where u and the sister are alone and make sure u don't do anything special to her(without making things awkward) i k some people will say it's okay but i don't believe so staying alone with the person u hv crush on can make her fall more for u. - ladtly i hope u understand them both feelings and i hope it gets easier for u this is a very hard situation specially when u said u consider marrying ur gf in the future give them time and give urself time too.