Would inviting them be the kind and appropriate thing to do? Of course it would be! But does she have to invite them? Sadly no, she does not, and she is the one who gets to decide. We can't always make people do what is right and kind. I can speak from experience here, my step sisters (who were adults when my mom and their dad got married) have always been awful to my mother, always thoughtless, and sometimes downright mean. There is no reason for this other then that they don't like their dad splitting his attention away from them. And while I have always invited them to events and send holiday cards and the like they have never reciprocated in any way. In the end it has cost them, not my mother. My step father is not blind to their treatment of the woman he loves (as I am sure your fiancé is not) and has started doing things to protect my mother from his children, which means his children end up losing out on times we could all be together. It is sad, but it is the path they choose. Just be the bigger person and always remain kind yourself. Best of Luck!