Funnily enough I was having this same discussion with myself last week. I generated a bunch of different options - most of them far too wordy.
At this point I want to go with whatever we all agree would be best here. I had the same initial thought as DZ realizing that Leia should know something was up with Palps if she knew Rey was one. But yeah, Leia would just have the same thought that we all had after seeing the film and before the novelization - that Palpatine had a secret love child during his reign. And he could be completely dead even if it meant he had an heir.
I’m wary of having it possible to sense somebody’s genetics purely by being around them. Luke and Leia definitely realized they shared some sort of connection in the OT, but it took Kenobi spelling it out for him to realize it. Especially since being a Palpatine is something so out of left field. Then again, Leia would have spent plenty of time around Palpatine in the Senate. I could buy it if it was more of a vision-like thing instead of just feeling it, but there’s no real way to write that in a concise way.