Stepkid Driver's License: Who Pays Insurance?

I think my stepdaughter just got her driver's license or is about to. I would hate to be the home to have to get that auto insurance quote right now. Technically, she could have received her driver's license a year ago but for whatever reason, her mother didn't get her scheduled and tested. 

She doesn't live with us so that's one bill that we don't have to deal with right now (I think). In three years, we will for our oldest daughter but for my stepkids, I believe that it falls on the custodial parent to provide insurance.  I think that is how it works anyway.  It better.  If my insurance goes up for a child who has more important things to do than spend time at dad's house, I'll be beyond ticked off.