Husband had an affair and now a baby with my baby sister

My sister moved in with my husband and I a little over a year ago because things at home (she lived with my parents) were getting really stressful. I welcomed her happily, she is the sibling that I am closest to; instead of paying rent, she offered to help me take care of my 3 young children, 5 and under. After a couple of months I noticed that she and my husband were overly friendly but paid it little mind. After a few months passed, I was out with friends one night and not due home until the next morning; I was unable to sleep well and decided to come home early... to find her asleep in my bed with my husband! I asked how long it had been going on and he said about 6 months. He moved out a few days later and she was kicked out immediatly.  Two months later, I found out that they were expecting a child and would be moving in together. After going to court, I was granted custody  and he got sunday visits. The baby has been born and now my older daughter (6) isn't comfortable going to his house for visits, I don't know what to do. She skipped her last 3 visits but he reminds me that I am in contemp if I don't send her since visits are court ordered. Also, how do I react to this situation? She is my sister and I have to see her often and he is their father so we too see each other often. I want the kids to accept their new brother/cousin because there is nothing that can be done now but I also don't want them to think that what their dad and aunt did is okay. Any advice please