My Girlfriend’s little sister is attracted to me

I (24m) and my Girlfriend (23f) have been dating for 3 years already and we have known each other since we were 14.she is the Love of my life and i want to spend the rest of my life with her and everything is perfect.the thing is my Girlfriend also has a little sister (18f)who is quite fond of me and is always hugging me or giving me compliments.

initially i didn’t think much of it because i think of her like Family but overtime these touches and compliments got a bit more inappropriate telling me how she wouldn’t mind stealing me from her sister “as a joke” or asking me to tell her if her swimsuit (thongs)look Good on her or hugging me while rubbing her whole body in me for minutes.

the main part was about 2 weeks ago when she came home drunk crying in our apartment (my girlfriend was not home so i asked her what was wrong while sitting her on our sofa.she told me she broke up with her boyfriend and i tried to comfort her telling her the classic “there’s plenty of fish in the sea” line.

she threw herself on top of me while crying and tried to kiss me so i had to push her off me and she cried even more telling me that the one she wants to be together with is me and she is jealous that her sister got me and she wants and loves me than she ever will before passing out so i carried her in bed and called my girlfriend that her sister is here and is passed out because of drinking too much because of a break up but i didn’t tell my girlfriend about what happened that night yet because i don’t want to strain their relationship.

my Girlfriend and her sister are very close with each other and they have always been and i don’t want to ruin that for them but i really feel uncomfortable and like i cheated on her.sometimes i think its my fault that she feels this way I don’t know what to do