I was engaged to one of my best friends sisters. She was the same way with me before I got with her.
She was a HB9. Great looking gal, we had the same hobbies, personalities, and so on, except she was crazy. In fact, before she met my parents (Hooking up for six months) she wanted to get married!
How is that for breakthrough comfort?
Back to your issue...
She is showing ioi's. So, as aforementioned, it is a GO!
Being direct is a no-no. I would still run normal attraction game. Kino is HUGE for this type of thing, because with a friends sister, things need to just happen, or seem that way. You cant make a move as it can ruin a friendship and make things weird.
Go in socially, playful kino and attraction material. Tease the mess out of her. Qualify her so you have a reason to 'like' her and kino until a move is a natural progression.