Is it possible to be friends with my ex girlfriend?

Normally, I wouldn't care whether or not I'm friends with an ex, but we were extremely close. I've tried calling a couple times before, but nothing good came of it.I just need some help on what to say to be a friend to her and not start arguing about what happened between us.Oh! And get this. Her older brother is going to marry my older sister (doesn't make us related)! I'm a groomsman and she's a bridesmaid and I have to escort her down the aisle in 2 months! So I would kind of want to be a friend and not someone she hates. Plus, I still think she's pretty cool.Please help!