Marriage it's over

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Ok this is to vent since I haven't talk to anyone about this. Just found out my husband had over 50 pictures  of pornography photoshopped with one of my sisters face. He swears nothing ever
Transpired between the two. I can't ever tell her about this as is not only sick but embarrassing. I do not want anything to do with him, this is beyond sick. 
He had the odasity to say if it was the other way around he would forgive me. I told him good for him but I've made my decision. 
We have a 5 yr and a 1 yr old. We are supposed  have my sons 1st bday party on Saturday and I don't know what to do.