Take it from a new wife who is dealing with this issue. You need to cut your ex-wife off!! Surely you must have some sort of parenting time set. If this is the case then it's pretty simple, You should have the children during your parenting time and if your ex-wife chooses to leave the children with other family members or friends during her parenting time then she's only hurting herself...
Your new wife is going to end up resenting not only your ex-wife, but eventually you and your children.