Why does Crystal allow Arleen to stay in the apartment in Evicted by Matthew Desmond?

Before Crystal moved into the apartment, Arleen and her two children were the tenants. Arleen's mother died and finances became tight, ultimately leading to eviction from the apartment. When Crystal moved in, she knew she could use some help paying the rent and she felt sorry for Arleen and her family. She decided to let them move in so they would have a place to stay and help to pay the bills.
Ultimately, they were able to keep up with the rent but angered their landlord by interfering in the lives of their neighbors. Despite Crystal's best efforts, both she and Arleen were evicted from the apartment once their landlord realized what they were doing. She didn't like the attention the apartment was getting because of the constant calls to the police.

Crystal allows Arleen to stay in the apartment with her two children because she needs help paying the rent. Since the landlady demands that the rent be paid on time, and Crystal needs a place to stay even though she doesn't have a stable job, she can't risk an eviction. She sublets the rundown duplex to Arleen on the condition that she helps out with some of the living expenses. They do have a few arguments every now and then because of Crystal's psychological disorder, but other than that, things go pretty well. The arrangement lasts for a short time before the landlady evicts both women for interfering with her business. As it turns out, Crystal was fond of calling the cops on a neighbor that used to abuse his partner. Sherrena, the landlady, doesn't like that, and she decides to evict them both.
https://monarchhousing.org/wp-content/uploads/policyforum/Evicted.pdf

Crystal's apartment is the place where Arleen used to stay and from which she's recently been evicted. The property is part of Sherrena's extensive portfolio, and it is she who evicts Arleen and her children. Arleen had fallen behind on her rent after borrowing $320 from Sherrena to pay for her mother's funeral. Sherrena expected that Arleen would receive assistance from either her family or from the state, but as no such help is forthcoming, the decision is made to evict.
Crystal offers to let Arleen and her children stay in the apartment and they become roommates, even though, as Crystal admits, she doesn't know them from Adam. Crystal has a number of mental health issues, including a very low IQ. She has a trusting nature and so has no hesitation in helping out a complete stranger.