I like my best friend?

i am girl and i am soooo in love with my best friend who is a girl. I think i am bi but i dont know for sure. I have invited her to come with me to my caravan fro my b-day in 3 weeks but we are going to be sleeping in a 2 man tent, wenever i see a pic of her or wenever she laughs i get the wierd feeling inside like we are s'possed to be 2gether. but she is straight and i dont wanna tell her that i luv her be4 she comes because she has already said that she is going to come with me!! i really luv her and i cant lose her. If u think i should tell her then what shud i say. She is like a sister to me and i dont wanna ruin our relationship please help me.