Affair Right Or Wrong

Side 1 says... The wife had an affair year ago which resulted in a baby. We stayed together, have 4 kids, two houses. She has stayed in contact with him as he has custody of the daughter. The wife finds no harm in taking our children over to the jackass's house and letting our children play with their step sister. She also brings the child to our marital home to play with my four kids. She has taken our two smallest children to see Santa with this jerk and their daughter. My nine year old son has asked how this daughter can be his step-sister. He doesn't under stand why she has a different dad then our two younger kids. She has been in contact with him about a job over the past several months. I have have pestered her about whether she was involved with him again but she says I have nothing to worry about. Well I went oversease for a couple months and found out some stuff. I found out that she has been sending him dirty videos and emails. She has has sex with him. She now works for him directly as a contractor. She drives to his house in the morning to commute in but claims its to meet other employees. Is she right for doing this to our family?