They've been together for 10 years and you've known your SO forever but only became romantic 5 months ago. You tried to be friends in the beginning - 10 years ago? How have things been for the last 9 and a half years of knowing your SO as friends. Was it just easier to be her friend before you began dating him?
I'd start by asking him to stop sharing so much personal info about her with you. I'm a bit confused as to why you'd need to know what manner of undergarments she wears unless you innocuously discovered it some other way during when you were just friends with your SO. If it is something he is instead telling you all gossip style, that should stop because its clearly creating a sour association for you. You're his GF, not his therapist. Time between the two of you shouldn't be about burdening you with issues he should speak to her about. It stinks that sometimes there is spill over when you start seeing someone who already has an established relationship. I'm sure you knew there would be some of that going in but you can ask for it to be minimized to only what you need to know if it would help you.
It might be best to try to remember that who you're dating is encouraging the very thing that irritates you. You can speak to him about that and then you won't have to deal with someone you don't like. Taking it to her instead of who got the situation started comes off a bit like you just want to be able to tell her how much you don't like her. What will that solve?
Are you unable to choose to not be around her?