From what I gather
She's already their property due to the contract
And he can't fight due to not having the right to do so because he's not an adventurer
So one of the only way out I can see is to buy her back
I'm kinda suspicious of Uichi's dad... is it possible he could have had an affair with Yotsuha's mother? Cause his story kinda ended a bit... anticlimactic
The sister got a job
The mother showed remorse
That dad though... nothing
Hmmmm