How do i tell my friend that her senior sister wants me to date her?

She's being playing her pranks for some times now, but i pretend as if i dnt knw what she meant. She buys me wrist-wathes, shirts and undies and talk me into dating older gals but i act as if i dnt understand what she's up to.. Last week, she told me she's in love with me, not because i'm a friend to her j.brother but bcos i move her hrt anytime she sees me. She told me she knew it sounds crazy but she mean it. I try to ask her abt what my friend would think but she said he will understand as time goes on. This is really weird, isn't it?
She's 3yrs older than me. Am really confused right now.