What's wrong with the mother that custody of the child should be changed?
That is the question your husband must answer and convince the court.
Unless there is something seriously wrong with the mother, you as the stepmother likely has no legal standing in this case. (Of course, in deciding whether to grant custody to the father, the court may consider your relationship with the children.)
Your husband should review his facts and options with an attorney.
first of all, if your husband has a parenting plan, the only way to modify it is if you can prove a substantial change of circumstances in his ex's household. this would be something egregious, like the children have started to drop out of school and do drugs, or maybe the ex. has become a drug user or a prostitute.
if the situation is simply that your household has stabilized and become more child friendly, that isn't good enough.
second, maybe there isn't a parenting plan. if that is the case, you need to file a petition to establish one.
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