That doesn't even make sense but then again it's Marvel comics so who the hell knows. Like are they not Magneto's children anymore?
No they're no longer Magneto's children.
Also who does Viper and Firestar belong to?
Viper was featured in a FOX movie and was referred to as a mutant... so how come FOX has rights over Viper when she is supposed to be associated with Hydra?
I think in Viper's case it's a borderline situation like the Maximoffs because she is indeed featured in a Fox movie.
However, she is not and never has been a mutant in the books. There's also to note that the character appears in the MCU-based show Agents of SHIELD, which increases the likelihood of her being in a similar boat as the Maximoffs - she is a Hydra character and a human in the books, but since she is so heavily associated with Wolverine, Fox was able to use her. Kind of a reverse on how the twins worked out.
What about Firestar? She is classified as a mutant but she is mostly known for appearing alongside Spider Man. Can Sony use her?
Firestar was created as an original character for the Spider-Man & His Amazing Friends cartoon because they couldn't use Human Torch for some reason (iirc he was dead at the time and Marvel didn't want to promote a dead character).
When she was brought into the official Marvel comic universe, she was not coupled with Spider-Man all that much, iirc only to tie into the show for a couple of issues, and mostly has been hanging out with Emma Frost and her Hellions (the Hellfire Club's variation of the New Mutants), and after they got wiped out and Emma joined the X-Men, Firestar over the course of her history was a member of the New Warriors, the Avengers and a tutor at the X Academy.
So she mostly is associated with X-Men properties in the books.