The brothers treat this girl better than their own kids, hope it's not just because they see her as useful property
It's an interesting situation because while I don't remember if anything is said about how common wizards are in this setting she is a highly skilled individual who can also do some highly unique things which means that regardless of actual employment/ownership the brothers must realise they can't actually force her to work for them.
Especially now that she's been invited to do work in the royal palace they kind of need to treat her well if they want Frim to choose to stay with them in favor of potentially better contracts by higher nobles.
It's suffering from success where if Frim had actually been a debtslave instead of a free woman the brothers would likely have been presented with an offer they couldn't refuse and now because the choice is in her hands they just have to hope they treated Frim well enough she'll choose them over being employed by royalty or going lucratively freelance in a couple of years.