The ex still being in the picture?

I have been talking to this girl for over 6 months. After returning to school from summer break we started dating and hanging out a few times a week and always on weekends as our busy schedules permit. Everything has been going really smooth we always have a lot of fun when we hangout, we have both planned little surprises for each other. I Haven't met her parents yet, but she is a girl who is very close with her parents and she tells them what is going on and her parents always ask questions about me. The only thing that concerns me is that she still has a lot of contact with her ex from high school. Now my girl friend and her ex boyfriend broke up in high school and Haven't dated since then. They are both now sophomores at my college. They both have a few classes together so I know I always see them walking to class together. I don't know if they hangout together or really do anything together, but from the notions I have got they really don't do anything together anymore, they aren't friends on Facebook. What adds another layer is my girl friend's cousin also goes to my college and he lives with her ex boy friend on campus. So I see the 3 of them together every so often too. I know that this girl I'm seeing and her ex talk everyday and seem to be pretty good friends still. I know when me and this girl are hanging out together and I see that her ex has texted her normally she doesn't even worrying about answering it, on occasion she will answer and maybe make a comment about like what he said or laugh. I have said like what before, and she tells me so its not like she is hiding anything from me. When I see my friends around they asked me if I was still seeing this girl because they made the comment they see her with her ex walking to class all the time. I want to go full hearted into this relationship and as I said it seems to be going really well. I don't want to listen to what my friends are saying, especially if it is just putting stupid ideas in my. I have no reason not to trust her because she has never done anything to me to think otherwise. I guess that was a really round about way with all the background information to, is it normal for a girl to still have that close of a relationship with her ex? Is it a possible big red flag that they still talk everyday or since its been 2 years and I feel as though a girl wouldn't be talking to her ex and dating someone with how involved her parents have been in the relationship is it normal. I'm just looking for insight from both sides and what people think of the situation. I'm just confused right now.

Also the ex boy has had a girl friend for about a year now