She’s jealous that Carly has ACTUALLY got married and now actually has a husband and is now a wife, because she KNOWS that she’s never going to marry Steve; there was no wedding booked or planned a few years ago - the whole thing was a lie, and now she’s yet again lying and pretending they are getting married next year but it’s all made up - it’s fictional. There’s no wedding in the future for Rebecca, no big day, no white dress, no husband. She knows it’s all fake and she’s jealous that Carly had executed the one plan Rebecca can’t.