When you are married to your children's other parent, you have a relatively large degree of control over the people your children are exposed to. However, after a divorce, it is typical for that control to decrease. It can be frustrating, as many parents new to this situation wonder if they can block their children from seeing certain people and find out that no, they pretty much cannot.
Why is this so, though? For example, if your ex's mother is domineering, why cannot you get a court order to block this woman from being around your child on your ex's weeks?
Each parent has latitude
In general, each parent has a lot of latitude to make parenting decisions. That means there may not be much you can do about your ex's relatives or romantic partner. On the other hand, if your ex dislikes the person you are dating, there is not much that he or she can do either. It cuts both ways.
The legal system presumes that absent extreme circumstances such as abuse or overt alienation, both parents are fit to rear their children the way they would like.
A matter of practicality
Another thing to keep in mind is that unpleasant people or people you dislike are a big part of life. Rather than trying to interfere with visitation or custody or to shield your children from these people, it may be more useful and practical to equip your children with coping tools and to communicate openly.
Communication with your ex
This can be easier said than done, but communicating with your ex can sometimes do wonders (and other times, it is quite frustrating!). For example, if your ex's mother often criticizes the way your child dresses, you can give your child or your ex strategies for how to react the next time this happens.
That could be more effective and legally practical than you demanding that your ex block his or her mother from seeing the child. However, in some situations, it can be reasonable to seek a lawyer's opinion. Do this if you are in doubt.