Do u think what my sister has done is out or order?

well my sisters little boy is the same age as mine they are both in the same school, a few months ago my husbands ex brought her child to the same school which our children attend and her son is also the same age and in the same class as my nephew. i wasnt very happy about it but wouldnt stop my son playin with him( its not the childs fault) .. however my husbands ex has put us throught years of misery and she has also cost him his eldest son whom he hasnt seen in years and has missed out on his child hood which even to this day kills him inside. well my sister is now gettin pally pally with his ex knowing full well the amount of **** shes caused over the years and all i get from her is "i cant help who my son plays with at school" .... am i over reacting ???? i dont think i am, all i know is if the roles were reversed i wouldnt do that as i know were my loyalties lie.