[footfootfoot]

Handling (hosting) these sorts of things looks a lot easier than it is. A lot of folks who attempt this are way out of their depth, this seems to be the case here. A good host/hostess would enlist the help of a reliable person(s) from "both sides of the aisle" to ensure names and other particulars are correct. Then there is the reality that everyone needs to be invited unless there are truly extenuating circumstances. Boors and gossips need to be suffered, but junkies who will steal things do not (for example)

As for what to do, how close are you to the couple? How much does it mean to your sister? As always, taking the high road, like good manners, never goes out of style.