The Star Wars: The Rise of Skywalker novelization seems to confirm Jannah is not Lando Calrissian's daughter. Viewers speculated the two characters were related in the months leading up to the film's release, but the movie didn't address the topic one way or another. In fact, fans had to turn to Rise of Skywalker tie-in publications like the visual dictionary to learn Lando had a child in the first place, another illustration of how The Rise of Skywalker is frustratingly reliant on outside sources to flesh out plot points and clear up any confusion. In that book, it's revealed Lando's daughter was taken away at a young age as the First Order looked to hurt old Rebel Alliance leaders.
Lando and Jannah have one interaction with each other towards the end of The Rise of Skywalker, as the Resistance celebrates their victory in the Battle of Exegol. During their brief talk, Lando offers to help Jannah find where she is from. Some audiences were perplexed by this scene, particularly since it was confusing what exactly was going on. People couldn't tell if the movie was revealing Lando and Jannah were father and daughter, or if something else was going on. As it turns out, it was the latter, and it appears fans haven't met Lando's daughter yet.
The Rise of Skywalker novelization, already infamous for its additional content, fleshes out Lando's scene with Jannah. Author Rae Carson taps into Lando's mindset as he figures out what he's going to do now that the war is over: reunite lost children with their families.
"Lando and the Lady Luck would help these special kids. Find their families, if that's what they wanted. Help them discover their new place in the galaxy. Heck, maybe he'd find his daughter. Probably not; he knew the odds. But that would be a pretty good way to spend his twilight years, right? If the kids were amenable, anyway."
The Rise of Skywalker novelization implies Jannah simply reminds Lando of his daughter and the two aren't one and the same. Jannah was kidnapped by the First Order and forced into service as a stormtrooper, so it's easy to see why Lando sympathizes with her and relates to her pain. Even if he can never reunite with his own daughter, Lando doesn't want the other families the First Order destroyed to continue to suffer, so he sets forth on a new mission. It's a very noble act on his part and shows how much he's grown as a person since his days as a scoundrel. Much like his old buddy Han Solo, Lando became very selfless in his old age, understanding the value and importance of love and family. He knows he's hardly the only person affected by the First Order's tyrannical approach to building a military, and wants to help others who were devastated.
Of course, it's entirely possible Lando and Jannah are related and just haven't discovered that yet (Star Wars is never afraid to retcon characters' lineage), but it looks like right now Lucasfilm is operating under the assumption they are from separate families. After all, the Rise of Skywalker novelization is the same book that went so far as to confirm Rey's dad is a failed Palpatine clone, so it seems reasonable to assume Carson would confirm a Lando/Jannah relation if that was the intention. Most will probably be relieved to learn the two aren't father/daughter, but it's something else that would have been nice to see covered in the movie, rather than a book only a fraction of the audience reads.
Source: Star Wars: The Rise of Skywalker novelization