I am a divorced (really remarried) mother of 2 and my ex has been married to 2 other women since me. It has been really hard on my daughters especially my youngest who is 9 now because she is a daddy's girl. IMHO, I have always let my ex make that decision. We are able to talk to each other pretty openly so I tell him how my daughter feels about not wanting to be around her step-mom and so not wanting to see him. Then I ask him what he wants me to do.. I will make her if he instists but I will also explain to her that it is his choice and I have to according to the law. He has always told me to let my daughter do what she wants, but to tell her that he really wants to see her or he calls her and tells her how much he would love to see her or how much he misses her. I can't make her like going and I can't make the relationship between them work, so I put it all right back on him. I am sure that your sister could do something very similar. Make him decide.. It is his decision to make and if she has a standard custody agreement it says that if she keeps her daughter from seeing her dad against his will she is in contempt of court the whole "I have to according to the law" is not a lie.
This would also depend on how old the child is. The older she gets the less i press the issue of making her go to his house (although he is getting divorce again so she loves going to his house right now.. no step-mom
I hope this helps a little. If I were your sister I would try my hardest to make it his decision not my own.
Good Luck it is a sticky situation.