I'd have to play through again for certain, but..
I believe Anna hasn't met Gardevoir in the story, so I don't know how that works?
And I do believe Radomus's knowledge of the Pendant comes from his broad knowledge overall, of history and whatnot.
Somehow I'm not sure what to think of it. With Radomus' main Pokemon being Gardevoir, a Psychic/Fairy and Anna and Noel's main Pokemon being Gothitelle and Clefable respectivily also being Psychic and Fairy that could be a link.
But you'd maybe have to dig into where the characters where originally from, which is the Online Reborn League, where most character development found place I think. It's hard not to believe that the characters we see now have evolved way more than their original forms in the League, where they were voiced by real people. This could've cause some development over time to shape them differently to what they were initially planned and their stories might've intertwined into relations.
But you'd have to ask the really old members for that I think.
Other than that we might get to find out if Radomus really is their father or not, and if not who it is as the story of the game progresses. Ame could be building up something and you expect what's coming next and then a complete turn to something even more horrendous.
Example: You have a lot of candles burning in your house, and someone comes to visit. They say: "Oh my, you have a lot of candles lit, that can't be that safe" and you think: "Yeah they could burn the place down" but instead they say: "I read somewhere that all those fumes from candle wax can kill you".
Ame might do just that.