Let's think about this from Karen's perspective. What has she done wrong?
After Cerys was kidnapped by Gaia, she went to the next best option to take our place. She had no reason to think we would come back, and no real steps to take to actively get us back. After that, we're really angry. For, uh, reasons? So we want some kind of guarantee that our sister will be protected even if we die. The best promise she can give is a magical oath, which basically means that she has to be telling the truth when she makes it. That she does plan on keeping it. Hell, she probably planned on doing it even before we asked- I could absolutely see the her training an orphan up as a support member of the organization, who would get to do a bunch of low-risk jobs while learning magic, with the organization ready to defend her like any other member.
What she gave us is not insignificant, and I don't think she could have possibly given us any better assurance than she did. After all, she gave us a magically-backed statement of intent.
I think she's entirely trustworthy regarding her interactions with us. And only us. Yes, she obviously wants our trust. It would help her, whenever some random person with an obvious agenda casts aspersions on her character, making it so she doesn't have to drop whatever she's doing to go get accused of being a liar before she can defend herself by some unhinged, overpowered idiot.
If we didn't choose to learn about normal magic, and instead trained up our Gaia bullshit, that's our choice. We did that.
The only thing that would be a problem was if she actually used mind-altering magic on us. Which is why I want to ask about that, under a magical oath for truth, after letting her get her two cents in. Probably in the update after.