If she is as attractive as you say, she is not liable to be impressed by yet another solicitous guy. I wonder why she was so friendly to you in the first place.
Also don't underestimate the influence of sisterly competition. It's possible you were just a bit too receptive to the younger sister. The older one may or may not have an issue with her sister but one way or another you can bet she was watching your interactions closely. Maybe she didn't like what she saw.