Susan Welker, portrayed by Laurie Metcalf, is Lois' younger sister and the aunt of Dewey, Francis, Malcolm and Reese Wilkerson. Lois hates her mostly because she's always been Ida's favorite. She's been mentioned several times, but first appeared in "Lois' Sister".
Throughout all her childhood, Susan has clearly always been the family favorite, being the pride and joy of Ida and Victor. She was the envy of Lois who learned to despise her. Susan was easily more talented and popular than Lois. She was often called on a lot of times by Ida whenever she needed help. In the first part of Baby, Susan had a brief misunderstanding with her mother that led her to move in with Lois for a while. Piama later had the idea to trick both Susan and Ida by telling Susan that Ida was dying and handing the phone to Ida to clear up the misunderstanding. At this point, Ida left the Wilkerson household to move back in with her favorite daughter. In "Garage Sale," Susan gave Lois hell for being hopeless and untalented during her visit and it made her reflect on how unmotivated Reese was (although she later realizes just how much he doesn't care and gives up on him). It was also implied in Family Reunion that Susan is the reason why Hal's family along with Victor and Ida resented him for marrying Lois over her
Susan finally makes her first appearance in season 5 and despite being cordial to Malcolm, Reese, Dewey and Jamie and giving them presents, she was very bitter with Lois for years. This in turn lead to a huge fight between them when it's revealed that she still blames her for stealing Hal away from her and it led to her unmarried status. Susan also mentions to the boys that when she dated Hal, his family approved of her despite her lower background due to her being higher class and talented. She mentions Lois took the animosity too far when she stole Hal from her, causing the friction between their families and Ida blaming the latter for humiliating them. She admits to it and refuses to apologize for what she did because unlike Susan, she understood Hal better. When Lois learns that Susan is dying of kidney failure, she offers one to her. She initially refuses to take one of Lois' kidneys and states that she'd rather die. Susan also believes this is a pathetic attempt to get ahead of her because Lois still resented her for having their parents' favor. Lois pleads with her to accept one of her kidneys because despite their past problems, she still loves her. Susan ungratefully and begrudgingly takes one of the kidneys.
In Victor's Other Family, Susan was implied to be Lois' half-sister, when Lois takes a DNA test and reveals she isn't related to Victor at all. Lois asks if Susan is Victor's daughter, Ida replies that "anything is possible." Although it's hinted that Ida could've been lying to her all that time
Susan was mentioned in Season 6's Living Will, when Lois snapped Hal of his fake paralysis and told him he made the right decision to marry her over her sister. She was both families choice of wife for him because she was high class and talented.
Susan was mentioned in the last episode of the series Graduation, discussing with Lois on the phone cause she was not invited to the Malcolm's graduation. Lois says the her mother "bullied her" into giving her the last ticket they had for graduation (this possibly in order to see how Reese screwed up the graduation's ceremony with his prank), implying Lois wanted to invite her. This implies the two may be on better terms now, although it could be that Lois is lying about it. Maybe Susan still refused to go and gave her ticket to Ida since she's still resentful over being cheated of death.
- It was revealed in season 4's Garage Sale, Susan was more talented than her and was always encouraged to do better. She gave Lois a hard time about it and it was reflected on Reese(though it was later revealed that he doesn't care about being unmotivated and is almost just as greedy as Ida).
- In Season 5's Lois' Sister, it was revealed that Hal Wilkerson used to date Susan and that both families wanted him to marry her. However, he chose to marry Lois against their wishes and it lead to conflict between Lois and Susan for years.
- Family Reunion supported how angry both Hal's wealthy family along with Victor and Ida were in him marrying Lois against their wishes for him to marry Susan.
- In Living Will, Hal finally admitted that he never regretted marrying Lois over Susan. To him, she was a big mistake on his part because she was insane and would never understood him like Lois did.
- There are many references in "Lois' Sister" that are a homage to Metcalf's performance in wikipedia:Scream 2, Susan is crazy as her scream 2 character Debbie Loomis was (though much more insane). Lois mentions that her bully, Sidney Bower, was beaten up by Susan to protect Lois. Sidney is the name of scream's protagonist Sidney Prescott and Debbie Loomis did try to kill her.
- In Season 5's Victor's Secret Family, it's confirmed that Susan and Lois are actually maternal half-sisters as Victor is not Lois' biological father. It remains unknown if Susan is Victor's biological daughter as even Ida is unsure though she likely is, given Victor's fondness of her. It's also possible that she's eligible for his pension and may be related to Roberta.
- In season 2's The Grandparents, the reason that Reese is favored by Victor and Ida is because his strong nature reminds them of Susan's own. It brought a sense of nostalgic feeling to them