Warning: MAJOR SPOILERS ahead for Fantastic Beasts: The Crimes of Grindelwald
Fantastic Beasts: The Crimes of Grindelwald spends a lot of time exploring the branches and flowers of the Lestrange family tree - but with all the movie's twists and turns, you could be forgiven for getting confused. In the movie, the orphaned Obscurial Credence Barebone (Ezra Miller) suspects that his true identity is Corvus Lestrange, and he goes to Paris in search of his mother. Meanwhile, Leta Lestrange (Zoë Kravitz) is hiding a dark secret about what actually happened to her baby brother, Corvus.
There are two families who play a key role in The Crimes of Grindelwald's revelations: the Kama family, and the Lestrange family. The Kamas are a family of Senegalese witches and wizards, and Yusuf Kama (William Nadylam) is the man whom Tina meets in Paris, who tells her about the prophecies of Tycho Dodonus. Yusuf is the son of Mustafa Kama and Laurena Kama, and his mother Laurena was bewitched and stolen away by Corvus Lestrange IV. Leta is the daughter of Corvus IV and Laurena, making Yusuf her half-brother.
The baby Corvus Lestrange V, who tragically drowned at sea, is also Leta's half-brother. Laurena died giving birth to Leta, so Corvus IV took a new wife, Eglantine, who gave birth to Corvus V. A visual representation of the connections can be seen in this limited edition print of the Lestrange family tree as seen in The Crimes of Grindelwald, sold by Minalima. Notice the portrait of poor baby Corvus V on the top right.
So, how does all of this relate to the events of Fantastic Beasts: The Crimes of Grindelwald? Well, Mustafa Kama was grief-stricken and furious when Corvus Lestrange IV stole his wife, so on his deathbed he made Yusuf swear an Unbreakable Vow to kill the person whom Corvus IV loved most in the world. Fortunately for Leta, Corvus IV didn't love his daughter, but he did love his son. So, Yusuf is in Paris trying to track down Credence Barebone, whom he believes is the lost baby Corvus V, so he can kill him and fulfil his Unbreakable Vow.
However, Yusuf's target is already dead. During a trip across the Atlantic with servant Irma Dugard (Danielle Hugues), Leta grew annoyed by her brother's constant wailing and decided to swap him with another baby on the ship. That baby was actually Aurelius Dumbledore, the younger brother of Albus Dumbledore. When the ship sank in a terrible storm, Corvus Lestrange V was drowned, but Aurelius Dumbledore was rescued along with Irma and Leta. Aurelius - still believed to be Corvus - was sent to an orphanage, where his name was changed to Credence Barebone. That's quite a lot to keep track of, but the main takeaways are that both Yusuf and Corvus V are Leta's half-brothers, Corvus V died as a baby, and Leta isn't related to Credence at all.
Leta Lestrange is distantly related (though not by blood) to Bellatrix Lestrange, who was played by Helena Bonham-Carter in the Harry Potter movies. Bellatrix was born Bellatrix Black, and was Sirius Black's cousin, but she joined the Lestrange family when she married Rodolphus Lestrange - who hails from a different branch of the family tree. So, Leta and Bellatrix's husband Rodolphus would be very distant cousins.