Ah, I wasn't quite clear in my post.

My wife changed her name in her koseki to match mine but left her
daughters alone.  Her daughter (a minor) was still living in Japan at
the time.  Now she has come to live with us and we would like her name
to match the rest of the family.

From what we've found, my wife's family can go to their city hall and
get the correct forms.  When my wife changed her name, but didn't
change her daughters it created a new koseki for her daughter.  Now
she has to apply for her daughter to be moved onto her koseki with her
new name - thus changing her daughter's name.

Does this sound right?

Thanks

Paul