Case of the perfect ex
Lucky you: your boyfriend's ex is the sort every woman would like to have, or almost...! Obviously it would be better if she didn't exist, but if she has to, you could do far, far worse. She knows you're with him now and she's part of his past. This kind of ex has moved on, made a new life for herself and is no threat to you.
They probably had an amicable split, which makes things easier for all three of you to stay on good terms. You wouldn't exactly say she's your best mate, but you don't worry about her, and every now and then you stop and chat and it's all very pleasant.
You're obviously a kind and understanding person if you get on with her. You don't get stressed out when you see her, and when you do see her you chat and pass pleasantries. You avoid talking about your private life with her because it would just be wrong when you know how intimate they were!
You have a rare and amicable relationship with his ex, which is a good thing and worth maintaining...as long as she doesn't start threatening your relationship, that is. It's unlikely with this kind of ex, but if she starts looking like a threat, simply cut your ties and forget all about civility!