"Humans, by their very nature, are possessive of the things they think belong to them. Some can be very intense and obvious about it, others hide their feelings away with logic and reason. But deep down, we all get a little nervous when the person we care about is spending time with someone else. Especially if that person is someone they dislike."
Sally hadn't assumed that Megan was jealous of Sally as a person. She was probably just disliked the situation, much the same was Sally did. In her mind, she couldn't see what Justin liked about her so much. Megan probably felt the same way about Sally.
She also knew that Justin would use that excuse when explaining why he moved in with Megan. "Listen. It's your life. I don't care what you do with it." Sally went to the cupboard and took out an unopened bottle of wine. Which she nearly dropped when she heard Justin's words.
"Well, if she thinks that, than she must be paranoid," Sally replied. Even she knew how head over heels Justin was with Megan - and she was trying really hard not to notice.