From the very beginning of Game Of Thrones, Cersei and Tyrion never quite got along, but the past few years have been especially tense ever since Tyrion was wrongfully accused of killing her son and then definitely did kill their father. Since then, she's talked endlessly about killing him, even going so far as to put a bounty on his head — but when presented with the opportunity to do so, something changed. On the Season 7 finale of Game Of Thrones, Cersei and Tyrion finally had a meeting since he first fled king's landing at the end of Season 4, and it was as complicated and emotional as you'd expect a Lannister family reunion to go.
Of course, this wasn't a social visit between Cersei and Tyrion; as part of his duties as the hand to Daenerys Targaryen, he set up a meeting between the two queens to convince his sister to put aside their dispute and join them in stopping the White Walkers — which she almost did, until she realized that with Jon on Dany's side, she'd be completely outnumbered. It fell to Tyrion to try to change her mind by appealing to her sense of family loyalty, which turns out to have been much stronger than either of them anticipated.
Once they finally ended up in a room together, all the; Cersei accused Tyrion not just of murdering their father (which he did) but also destroying the family entirely, as his meddling also indirectly lead to the deaths of Myrcella (who he sent to Dorne in the first place) and Tommen (who was the victim of his destabilizing influence). Tyrion attempted to apologize, but of course that's not good enough, so he told her to put her money where her mouth is: If she's so devoted to kill her, why doesn't she just tell Ser Robert Strong to kill him?
Honestly, I say this probably every season, but Lena Headey deserves an Emmy for the look on her face as Tyrion seems to call her bluff — and when he correctly identifies that she's pregnant. But just because she can't bring herself to give the order that would execute him doesn't mean everything is hunky dory; as she tells Tyrion, "Your love doesn’t matter. Your feelings don’t matter. I don’t care what you did, I only care what it costs us."
Obviously Cersei's going to keep nursing this grudge, even if she can't kill Tyrion herself; although she told Tyrion. Daenerys, and Jon that she will aid them in their war against the White Walkers, she's clearly already scheming to swoop in and take all the land that Daenerys leaves behind. But even though she might be "the most murderous woman" that Tyrion's ever known, at least it's clear that their family bond hasn't been entirely broken.
Or, at least, she's willing to set her immediate feelings aside to let Tyrion believe that he's won her over. As she said later in tonight's Game Of Thrones episode: No one walks away from her. Not for long, anyway.