Since the early episodes, Beth and Jamie have been at odds, with Beth loving and supporting her father above all else while Jamie's loyalties are much less clear. However, after the events of Season 4, their animosity has grown to epic proportions, becoming the rivalry that may ultimately define the fifth and final season of the show. This isn't entirely about their father, with the two sporting some clearly incompatible qualities, but sharing the same ruthlessness.
However, with the latest round of episodes, we've discovered that Beth's hatred of Jamie is rooted in their teen years. When she sought an abortion as a teen, he took her to a clinic that forced sterilization and knowingly made that choice for her without telling her until it was too late.
This has clearly caused a rift between them that has only gotten worse with time, and that's made all the more clear when Jamie gives Beth a ride from jail. She spots a car seat in his backseat and he tells her he has a son, which leads her to attack him until he's forced to pull over. This is a fascinating scene, as her genuine love for her brother shines through along with her profound disappointment in him for letting her down when she needed him most. Hurt that he now has a child after taking that possibility away from her forever, she tells him that she will take the boy from him, which leads to Jamie nearly gunning her down with his car.
