Given that there isn’t an animus gene in the Silkwings (that we know of), their power could come from several factors, and thats what makes this really fun!
Their egg could have been enchanted by another animus by the request of their parents (this in and of itself could be for many interesting reasons), maybe something they found gave them these powers, it could even be tied to something they wear that grants them animus abilities. Maybe they’re limited in what they cast or how they cast it, maybe they’re scared of their abilities and don’t want to use them, there’s so much you can do with that Silkwing!
And please don’t worry about Mary Sue stuff, I find that this really only applies to stories where the author really just wants to have fun in a world with their OC, and they don’t fully understand conflict, stakes, or how to build narrative tension, or they don’t understand why it’s important. There’s also the trend that in a Mary Sue story, the authors that write them tend to break canon characters just to have things their own way (having every single character they like be infatuated with Mary Sue, while the ones they don’t like are portrayed as evil and/or stupid, even if they’re not in canon).
I think the ultimate thing to keep in mind is the king that drives all stories and characters: conflict.
What problems do they want/need to solve, what forces are opposing them, what can they do about it, who would side with their decisions, who wouldn’t?
Sorry for the essay, I’ve actually thinking about Mary Sue’s recently, and realized some stuff about them (as you can see lolol)