Girlfriends 18 year old sister

It has been awhile since I have had a really great sexual experience.
I do have a steady girlfriend but she is the type of girl who will have sex but does not like it that much. It is not my fault, she just has some motional problems and is not a very sexual person. Don’t get me wrong, she is an extremely beautiful girl with golden blonde hair and an amazingly fit body. She is also great to be with but sexually, she is very frustrating.
I have been with her for almost two years and have got to know her mother and 18 year old sister very well. In fact her sister, Jaime, says she is totally comfortable with me, like a brother. I know this is true because of the way she walks around the house in her swimsuit without caring that I am there. (Most girls are very self conscious of their body
at that age.) It is hard for me to be just as comfortable when she does this because her thong bikini is very revealing. This wouldn’t matter much if she didn’t have a gorgeous body. But she does. She is about 5’8″ and 115 pounds. She has the most perfect long blonde hair and amazing tanned legs. They are so long that I can imagine the way the would wrap around me. Of course she doesn’t help my sexual frustration when she wears the tiniest shorts that just barely show her lower butt cheeks. This girl has got to be perfect.