My partners mother is friends with her x husband after 10 years?

I'm really pissed because after 10 year of being with my girlfriend I have just found out in the last week that her mother speaks to her x husband twice a week on the phone, and has his sister also goes round her mothers house as well sometimes. I am finding it hard to believe no one said a word to me about this 'friendship' for 10 years. it may be because her mother hates me as she use to tell my gf that I was waster and she should leave me and it's her way of getting at me, or perhaps it that her x husband way of trying to stay contacted to my gf though her mother, am I overreacting about this? is this completely normal behaviour??? or have they been laughing at me for 10 years???