Red flag?

My boyfriend’s best friend is a girl he used to work with, they’ve been friends for about 2 years. They talk almost every day and used to hang out a lot, until he met me. She has a boyfriend but I get the feeling that she finds my boyfriend more interesting than her own. When my boyfriend tells me about her and her boyfriend it sounds like she doesn’t really like her own boyfriend and that she lowkey wants my boyfriend. On social media she hasn’t posted any pictures of her own boyfriend, but there are 3 pictures of my boyfriend (before me and him got together). I’ve always been suspicious of how close my boyfriend was with her and i’ve told him multiple times to take a step back because I was really uncomfortable with their bond. He would talk about her every chance he’d get and I almost stopped dating him for it because he was giving her way more attention than he should have. He keeps telling me about how much he’s teasing and bullying her, like they’re siblings. He teases her for her height, body (she has the same body type as me, if not slimmer) and social life. He laughs about it and tells me that they fight and argue like siblings and that he loves to tease her. It still makes me uncomfortable and it doesn’t seem like he takes my feelings into consideration. Their relationship is so weird to me and he’s told me before that he would never stop talking to her, even if I wanted him to because she’s his “best friend” and she’s always been there for him. I don’t know if i’m overreacting or if I should talk to him again.