Secret relationship, how to be together?

so me and my girlfriend are in a secret relationship...dont ask why because that has nothing to do with it...only her sister knows...I see her every other day but cant get alone time with her anymore because her other sister is always hanging out with her...and they go to bed at the same time...help?

I dont want our relationship to go down the drain because we couldnt find alone time...and telling people isnt an option...

Why isnt telling people an option? If its that important to the both of you then it has to be an option. try and aks her to get her sister away sometiems, or just hang out with her and her sister and act as if yal are just friends.