Mrs. Van Daan is important as an alternate mother figure to Anne, as Peter’s mother, and as a sower of discord in the attic.
In their early days of confinement, Anne feels pressured by the amount of time spent with her own family. Although they have to make adjustments to share the space with the Van Daans, the novelty of having others around is appealing to her. Anne finds Mrs. Van Daan a reassuring presence and feels a kind of emotional connection to this outgoing woman that is different than to her own mother.
As Anne and Peter become friends, it seems that Mrs. Van Daan resents Anne’s attention to him. This in turn makes Anne uncomfortable, so she becomes more distant from her. In addition, Anne cannot help but be aware of the older woman’s pettiness as she takes offense at trivial matters, such as Anne spilling milk on her coat. In such a small space, Mrs. Van Daan cannot make the right kinds of efforts to ensure peaceful coexistence, and Anne comes to appreciate her own mother more.