Susan Welker is the younger half-sister of Lois and younger daughter of Ida. Before making her only appearance, Susan was mentioned a lot. She appears in season 5 episode 13. Susan was portrayed by Laurie Metcalf.
Before her only appearance, Susan was implied to be the family favorite. It was hinted as hard as Ida and Victor was in raising both her and Lois, Susan took it in stride and showed class for it. Her sadistic nature came out when she had discovered Hal having sex with Lois, an action Susan and Ida never fully forgave her for.
When Susan arrived for a visit, she gave generous gifts to Reese, Malcolm, Dewey and Jamie. Lois was suspicious of this and had confiscated the gifts. This lead to a fight and Susan accused her for stealing Hal away from her just because she wanted him for herself.
When Reese and Malcolm confronted Susan the next day, they had discovered that she used to date Hal and that both their families approved of them. Susan inferred that Lois was always jealous of her because she was everything she wishes she could've been and had sex with Hal to pay her back. Lois admits this, but refuses to apologize to Susan because she and Hal love each other.
When Lois learns from Susan's doctor that she was dying from kidney cancer, she secretly went through with the surgery and offered her the kidney. Susan refused to take it, believing Lois is trying to remain ahead of her. She announces her intent to die so she can prove her point to Lois that she is nothing more than a control freak. In that moment, Lois pleads with Susan to take her kidney and admitting after the trouble that she has caused, she still loves her sister. Susan begrudgingly accepts her kidney, even though she still prefers death.