All this occurred in a story I’ve yet to read. That being said, I ‘d say that Magneto made a very wise move entrusting Polaris as his heir over Quicksilver and Scarlet Witch. Polaris is just a much more even-tempered and pragmatic thinker compared to either of her siblings.
Quicksilver is impulsive, arrogant, and makes all sorts of poor decisions. I wouldn’t put Quicksilver in charge of looking after my cats, much less an entire nation.
The Scarlet Witch is much healthier now, but she has had a long history of psychological difficulties and she too has made many bad decisions (granted a lot of Wanda’s more destructive acts were due to writers utilizing what I consider to be sexist tropes about the dangerous nature of feminine hysteria).
In any case, if it were up to me I’d definitely choose Polaris and think Magneto made the right call.
What @supergirltastic said in her reply is definitely accurate when it comes to Wanda and Pietro’s attitudes, and I agree with her on the favorite child part even though it’s yet to be proven.
I don’t have pictures/scans of it, but the matter of Lorna as Magneto’s heir DID come up in the official comics during the Austen era. It happened in a discussion among the three of them post-Genoshan massacre. The twins’ attitude was the same as what supergirltastic said: did they really want to be Magneto’s heirs anyway?
Lorna also has a few other qualities and background in and of herself that make her the best possible heir for Magneto. While Wanda and Pietro tend to focus on Avengers stuff and rarely on mutants stuff, Lorna’s almost always dealing with mutant issues. In her best depictions, when Marvel cares about her and uses her right, she’s a very strong defender of mutant rights.
Meanwhile, on Genosha, she was very instrumental for Magneto’s reign. She pretended to be him for public appearances while his powers were waning (which takes a lot of trust; what if she decided to ruin his reputation out there?), and the Genoshan people looked up to her to such an extent that they were all idolizing her and begging her to save them during the Genoshan massacre. So, not only was she supporting her father’s dream, she was also an inspiration for the people while Genosha existed.
Lastly, her powers. It would be very easy for Lorna having the same powers as her father to lead to perception of her as “Magneto’s true heir.”
The other thing is the idea of Lorna being Magneto’s favorite child. I has headcanon about it.
My theory is: Lorna is Magneto’s precious baby magnet, and he wants her to have the best life she possibly can… away from the suffering he thinks he could bring her.
Magneto knew even while Lorna was a child that being in any way connected to him would put Lorna at risk. He got a firsthand example of that when Anya died. Same reason he took so long to tell Wanda and Pietro about him as their father, which is why it took Lorna finding the truth for herself before Magneto was willing to admit he’s her father. Also the Magneto that “disproved” Lorna as his daughter turned out to be a robot – it was Magneto covering up their family connection again to protect his precious baby magnet.
By the time Magneto started Genosha, I believe he recognized that Lorna would be at risk whether she knew he was her father or not. From there, it became less about protecting her and more about training and teaching her as best as possible to take over for him since she wanted to do so.