Waiting for a friends sister?

So I have a good friend and his sister and I like each other.I have really liked her for awhile and I've had like 4 dreams about her. I'm extremely close with their entire family cause I'm over there a lot but I'm not exactly sure how to act around her. Like do I always talk to her or do I be with my friend? I heard from my friend that she wanted to wait for us to date because she could see us getting married one day. Now I can understand that and I'm willing to wait but I'm afraid of her liking someone while I'm waiting or somebody else liking her and I just wasted my time liking her. This has been bothering me for a long time and I could use some help..