Early Morning Theory
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Radomus and Serra are revealed to be the parents of Anna and Noel. Serra and the twins didn't know this because Radomus (w/ assistance from Uxie?) wiped their memories of him and of each other to protect them from Team Meteor. Bennett didn't know about his half-siblings because Serra was rather promiscuous during her modeling days, and she kept their identities a secret from him with the hope that her son would make the right choices in the future if his mother set a proper example. Going back to Team Meteor, they were now seeking Radomus' pendant and blood both. Several years of his actions had finally caught up with him. You see, Radomus (just like Corey) became an agent of Meteor in an effort to impede their progress from within -- he knew exactly what kind of threat they posed to Reborn, and it was for that reason he chose to play white rather than black. At some point, he managed to gain the respect of El, who was the owner of the Amethyst Pendant before him. After they had done some more 'bonding,' El trusted Radomus enough to give him the pendant in a way that mirrored Solaris giving El the Emerald Brooch (the same one that cured his blindness). Radomus would eventually be exposed as a fraud though (possibly by another up-and-coming agent named Lin), and a manhunt began when word of his treachery got out. He was fully aware of how far they'd be willing to go to reclaim their lost key, and he was now on borrowed time. Ever the strategist/pragmatist, Radomus chose the best remaining path available to him. In sequential order: he gathered his family together, had Gardevoir use Hypnosis on them (putting them to sleep), purged all knowledge of one another from their minds, gave Nomos, Nostra, and the pendant to the twins, placed them at the Orphanage's doorstep under the cover of night, and turned Serra (plus any other relatives he had) loose when she woke up the next morning. Team Meteor's only concern is with collecting the four keys, so if they were to locate Serra and ask about Radomus and the pendant -- and when they see how clueless she is about what they're asking -- they would have no cause to threaten any lives. In a move befitting a Chess Grandmaster, Radomus sacrificed his own happiness for the safety of the people of Reborn. Pretty sad tale.
Side Notes:
1). I know Zina Vanhanen is a thing -- and while that could be a potential hole in this theory, I don't believe her relation to Radomus is ever stated. If she indeed is Radomus' dead wife, this theory would be completely debunked because of what he told Gardevoir about soulmates in his Interview Intermission.
2). Serra says at Voclain Estate, "I'm fairly certain that I've only had one child, and he's right here." The key word here is 'fairly.' If someone had one child, they probably would be more assertive than only being 'fairly certain.' Perhaps her deep subconscience survived the memory wipe and was on display for us in this scene? Also, she said at some point that she gave up modeling for Bennett's sake. Maybe she didn't mean right after his birth, but after she had been relieved of her ties to Radomus and the twins.
3). Although I say that Radomus gave Nomos and Nostra to the twins, it's entirely possible that he didn't know they were real Pokemon like many of us theorize. Anna is a different type of psychic than her father; more perceptive than procedural. Anna is to Radomus as Kiki is to Samson.
4). My initial explanation leaves this hole: who helped Radomus raise Anna and Noel? And to that, I say Serra did. She could have used many of her modeling tours to drop by Vanhanen Castle so she could fit in her share of parenting -- and Bennett would be none the wiser, since he was probably off studying butterflies anyway. I would think that Spinel Town's crime rate is very low, which means that Serra wouldn't have to constantly worry about Bennett's safety.
5). If you find any other holes in this theory, let me know.