Estranged Father and ex-husband in Cahoots

Winterville, NC

My father and sister have been estranged for over 1 year. My sister has been divorced for over 4 years, however she and her ex- husband have a very bitter relationship. Our father and her husband bonded because our father is a retired military officer and her ex-husband was going through commissioning as an Officer. She recently discovered from her 9 year old daughter that our father has been communicating with her by calling my sister's ex-husband over her daughter's court-ordered summer visitation. My sister is appalled, broken-hearted, betrayed, angry and more. Is she over-reacting? Our father was horrible growing up and put us through a lot of unnecessary drama and trauma and her ex-husband became a monster to her during divorce proceedings and afterward. She feels ganged up on and helpless. I am estranged from my father as well and feel he is dead-wrong. Any advice?