According to
IMDB's 2025 Fantastic Four page as it looks today, the called castings for characters are:
Galactus - Ralph Ineson
Reed Richards - Pedro Pascal
Sue Storm - Vanessa Kirby
Johnny Storm - Jospeh Quinn
Ben Grimm - Eben Moss-Bachrach
Shalla-Bal - Julia Garner
Other cast members (without character credits so far) include:
Paul Walter Hauser
John Malkovich
Natasha Lyonne
Vanessa and Joseph do look like they could be sister and brother, so I kind of get that casting. But they're both older than the characters were when they first appeared in the comic books. Not that comic book ages mean anything, but there's been a fair amount of speculation about how the team will be introduced into the MCU.
There's been a rumor circulating online that their story would begin in the 1960s and then be brought forward (just as Captain Marvel's story began in the 1990s and was brought forward). So it could be - if this is true - that the mature FF will have an encounter (maybe with Galactus?) that causes them to be "lost" for a few decades.
I just don't know if the time travel/retro plot line has been debunked yet.