Meh, she is not completely evil, with the brother for example, seems like he did have a bad relationship with his mom
She is evil, she decided to screw another family. She will likely destroy the sister and the mother.
+ She acts like a total hooker, this shit doesn't go well for children. Read some books about children who had mothers like that (not necessarily prostitutes but women who had many partners, even if those are secret rendezvous more than likely children noticed it. In other cases those partners abused those children or at the very least gave unpleasant and traumatic memories.
With the brother? She uses him and found a fissure that she could expand. Like a meticulous psychopath who found a weakness and exploits it.
More than likely she will screw the brother (by screw i mean destroy him). He will be sexually attracted to her but her daughter wants him, he on the other hand is cordial with the daughter due to her mother, the problem is that mother will try to seduce his father. If you don't call it evil i don't know what else can be called evil