My best friend crushing on my SO?

I may be reading into more than I should, however she constantly mentions the times they've had together or when they've gone out drinking. At first I wasn't bothered, it just seemed like friendly conversation and we've all been friends for years and she went out with his friend. 
Then as time went on she mentioned something I didn't know about... She'd shared her bed with him after a night out! I don't know why she randomly decided to spring that one me...
When his birthday came up it was like a competition for who knew him better with her. Telling ME what aftershave he likes (well duh I know that I live with him and I've known him very well for the past 4 years!) to the point she actually knew his clothing size and he isn't a common everyday average size. I just thought it odd. 
When we were out she kept touching his arm and talking to him and being around him whenever she could, and the drunker my SO got he kept putting his arm around people, including her. You could see how she was gushing over it... 
The way she is with his family drives me crazy as well and now they all constantly mention her to me including his older sister. Even my SO confused me with her as he mentioned to me he found it hilarious what his sister had said, only I told him she didn't say it to me and he said. Oh sorry yeah it was to 'best friend'. 
It's really put my guard up and I've been really off with him but I had talked to him about it all before but nothing has changed. The drunken events happened last night. 
Sorry for rambling I'm just a little concerned. Thanks for reading.