User blog comment:LegerPrime/Mother's Key/@comment-24228963-20151223090352/@comment-7418318-20151223110443

You don't need to justify for Brandish not using Celestial Spirit Magic.

The vast majority of mages in the Fairy Tail universe have to learn their magic, which means they not only can, but have to choose which magic to learn. Or course there are exceptions like Erza or Mirajane, who somehow instinctively find out how to use magic without ever learning it, but those seem to be the minority.

That some siblings have similar magic is due to them thinking in similar ways and thus are likely to make similar decisions, or due to "I want to do that too" after seeing their sibling or parents use a certain kind of magic. A prime example for the latter would be the Strauss Siblings, where Elfman and Lisanna learned their respective branches of Take-Over so they could be like their big sister.

While I wouldn't necessarily rule out the possibility of certain rare kinds of magic being passed down through blood-ties, we have yet to see any comfirmed cases of such happenstances (though granted, Erza and Mira might be just that, though we don't know). Most times when parents and children share the same kind of magic, it's solely due to the parents teaching their children how to use what they know, not due to some genetic inheritence.

Just look at Zeref and Natsu or Kagura and Simon or Wally and Richard/Hoteye. They are all siblings and still use very different kinds of magic from their respective brother or sister.

That being said, I don't think that it's too far or ridiculous. If the Spirits that originally belonged to Brandish and Karen's mother were passed on to the two children, and then due to Brandish's abuse quite their contracts and then moved on to Layla it could explain her hatred and Brandish might want to take it out on Lucy, which would be to take those keys back either due to Brandish feeling they should be hers or because she wants to punish the "traitorous" Spirits.

With the way she treated Marin and Lucy, abusive behaviour certainly doesn't seem out of place in Brandish's history.