<human>: Condense the text into a few key points

So I am dating this girl going on for about two years now and she is pretty nice and incredibly attractive but honestly we have near nothing in common and she has been disloyal in the past then swore to never do it again when I confront her about it. 

So with this all going on my best friend (I will refer to him as John) has a sister that is one year younger than us that is really attractive and we have a lot more in common and she is really cool! 

Also, she is really good friends with my girlfriend as of late which basically creates this massive loop of just off limits for me I feel like but I really like her and I feel like I would be much happier with her. I never thought like this before but this has been going on for the past several months and I just need to figure all of this out. 

I am kind of worried that "John" will be upset but not really because he is really chill and I think would be cool with it. The real problem is 1. I have no idea even what to do and 2. my SO and "John's" sister are kind of friends.
<bot>: I have a pretty big crush on my best friends little sister and she is friends with my girlfriend. I have no idea what to do!