What do you think? My new boyfriend has this galpal ....?

He calls her his sister, and his dad calls her his daughter. I have yet to meet her, but hear about how pretty she is and her straightforward personality...She's 2 yrs younger than him, he says it has never been anything ual but it just sounds so unlikely to me. They used to go up to her parents house in the country and just hang out-anyway now he says that her man is one of his good friends. What would you think/do in this situation? I am so conflicted I don't want to be hatin on her cause she sounds like a cool gal but I also don't wanna be taken for a dummy .....