Complex sentence: Her most despicable act was the murder of Doug's ex-wife, Eunice Gardner Wyatt (Ann Williams), Jo's sister, because she (Jennifer) was trying to steal her husband, John Wyatt (Val Dufour).
Split into simpler sentences: Her most despicable act was the murder of Doug's ex-wife, Eunice Gardner Wyatt (Ann Williams), Jo's sister. This was part of Jennifer's scheme to steal Eunice's husband, John Wyatt (Val Dufour).