Who do I escape it all?

Have you ever had a Bestfriend who has a really cute, the type you wanna date but cant even though you have had feelings for him for a while type of 'brother'?
Well I'm guessing you haven't, but this is something that happened and is still happening to me right now.
I've known him since third grade she's only known him for a few months, by the way not talking about the same she anymore the she I am now referring to is my sister. Well not biologically she's my step sister Lauren, the guy I was talking about earlier well that's her new boyfriend. Her ex used her to get closer to her sister Alexis my other step sister. Well either way now that they're dating I've been feeling a bit off and I've been faking a smile and faking my happy approval. I really don't know what to do in this situation, I don't know what to think other than 'she doesn't understand how my voice and my mood changes when I lie to her'. I'm not happy I'm sad some could say I'm somewhat depressed.