Panel 1: Annie tells Elly that if she wanted to look for work, she'd gladly babysit Lizzie; Elly says 'Really?'
Panel 2: Annie says 'Sure! I'd love to have her here (as opposed to having her parked in a day-care center) and it would give me some extra cash (which is why she objects to day care centers).'; Elly says that that might work out.
Panel 3: As she puts Barnacle Girl on the floor to play with Christopher, Elly says that the two of them are like brother and sister.
Panel 4: Lizzie reaches for Christopher's teddy bear only to have him yell AAGH and WHAP her on the head; as always, the force makes her stick out her tongue.
Panel 5: As she and Annie rush to pull Lizzie and Christopher away from one another, Elly says 'Literally!'
Summary: This is because of Lynn's assumption that siblings fighting one another is something that just happens and can't be prevented, only controlled. If it could somehow be preventable, that would mean that someone is responsible for making it happen and that someone would end up being her; thinking that she herself is responsible in any way for the chaos in her life is one of the unfair things she hates.