Game of Thrones is defined by royal families constantly vying for control of Westeros. Those families have all had their own conflicts and inner turmoils, but they've largely tried to work past those problems in the name of unity. With the White Walkers finally breaking through the wall and coming to destroy Westeros, it's going to take a unified front to have any chance of stopping them.
But long-standing conflicts within two of the most powerful ruling families could threaten to bubble over and bring about the end of hard-fought alliances. The Stark siblings and the Lannister twins could turn on one another and bring about the end of Westeros in the process.
The Lannister Twins
Jaime and Cersei Lannister have gone through a lot of changes over the course of the series. At the beginning of the story, the two were caught in an incestuous relationship. To ensure they weren't found out, Jaime pushed Brandon Stark out of a tower, inadvertently lighting the fuse that would eventually set off the War of Seven Kings.
But as the war progressed, both of them led ever-changing lives. Jamie was captured by the enemy and forced to reassess himself as the cold and efficient knight he believed himself to be. He lost his sword hand, formed a close relationship with Brienne of Tarth and began to recognize the value of others.
Meanwhile, Cersei has faced a different kind of harshness, becoming increasingly hardened by the ruthless politics of King's Landing. She's lost her three children to the machinations of herself and others, and while she became the undisputed ruler of King's Landing, it's a tenuous position that she maintains more through fear than anything else.
The two struggled to come to terms with one another in the final episode of Season 7. Despite a feigned agreement with Daenerys and Jon Snow, Cersei decided to not rally her forces to help humanity. Jaime tries to make the case for the alliance and convince Cersei to send their armies with him up north, but he fails. He rides off alone, without an army to fight against the White Walker horde. Cersei might finally turn on him for this betrayal, jeopardizing a potential future alliance.