After Elrood murdered Elliott, Barbara Mutelli went insane. Elrood repeatedly impregnated her, and then had the embryos removed and placed in storage for later dynastic emergencies.
Shaddam Corrino IV was born by Habla who was implanted with one of Barbara's preserved fertilized embryos. This makes Shaddam and Wensicia full siblings even though Shaddam is unaware of this and views her as a half-sibling.
Barbara's bastard daughter ended up living longer than her father.