My sister is having a baby by my ex-husband?

Today I just found out that my sister is having a baby by my ex-husband, who I have a six year old daughter with. I'm a little disturbed by this because for the past five months she has been lying to me. Only reason I found out is because my 18 year old daughter overheard her talking on the phone to him and she confronted her about the situation. Does anyone find this to be vile? I mean I was married to him for 5 years. I would have never thought my sister would do this to me. I just feel like slapping her, but since I'm an adult I'm not going t stoop to her level. I love that girl to death but she has done something that I cannot forgive her for. Should I just push her out of my life?