Lori Vallow was a woman who became infamous due to her alleged involvement in heinous crimes related to her children. She was associated with doomsday beliefs.
It depends on the context and development of the story. Sometimes love alone can overcome many challenges, but in other cases, external factors might play a role.
Another possibility is that one of the other characters in PLL has a hidden motive. For instance, someone who is jealous of Allison's popularity or someone who she had an argument with in the past. They might have found a way to make her sick without being suspected at first.
Lori can be developed by showing her internal struggle. Her jealousy could stem from her fear of losing Rick, so exploring her past relationship with Rick and how it contrasts with what she sees between Rick and Michonne can be a start. For example, if she realizes that Rick has changed and she no longer fully understands him because of Michonne, it can lead to growth.
Well, without more information, the story of Allison and Emily could be anything. Maybe they are sisters - Allison being the older and more protective one, and Emily the younger and more adventurous. They might have grown up in a small town and had to face challenges together like dealing with mean neighbors or trying to save their family's old house from being demolished.