What do you do about them? Pawn them off as cousins? As friends of the family? I mean, when one's family doesn't know that one's fiance has an illegitimate daughter from a teenage romance--a secret that hasn't been appropriate to share--what does one do when the fiance wants the daughter, now a teenager herself, invited to the wedding? How does she tell her parents without them feeling painfully betrayed? And what about the opinions they'll then have?
And what about the fiance's friends, most of whom don't know about her existence either? Does the happy couple suddenly tell these friends, out of the blue? Or try to bribe the love child and her foster family into acting like distant relatives? Distant relatives who happen to look very related.