ex-wife auto concerns...!

My husband's former wife is hell bent on allowing my teenage step-daughter to drive a car of her own. Now that her mother has several cars she wants to depict equal attention to both of her daughters. Incase she gets our daughter a driver's license, we need to ensure that she gets a proper coverage. Since we don't intend to get car jointly along with her mother, we are hoping to post a public notice displaying the conditions following which we could allow her to drive and also a choice of vehicles.
Please suggest a way out of our dilemma keeping with the fact that her parents have a joint custody. Thanks !