Other mistake: When the girls are sitting in the common room on the night Giselle shows them the contraceptive, Betty says, "Do you realise November 2nd is three weeks from now?" Yet, the University newspaper with Betty's nasty editorial shown on screen the next day is dated September 19th. (00:18:43 - 00:23:53)
Other mistake: In the scene where Betty and Joan are in the kitchen after dinner, we see the coffee percolating in the pot but never see Betty pour the coffee into the china pot before taking the tray into the living room.
Other mistake: Betty's mom is requesting a change in the table setting, eg. no candlesticks, glassware trimmed with gold etc. You then see the actual wedding happening and see candlesticks and glassware without the gold rim.
Answer: I can not find any reference to Joan Brandwyn's mother. I do not remember seeing her. Perhaps you are thinking of Mrs. Warren, Kirsten Dunst's mother in the film. She is played by Donna Mitchell. I have been unable to find a connection between the two.