One of the scary things that children think about when their parents go through a divorce is what is going to happen with their relationship with their aunt, uncles and cousins. This could actually keep the kids up at night, so these extended family relationships must be considered carefully when you and the other parent decide to call it quits.
It is imperative that both parents remind the children that they are going to continue their relationship with aunts, uncles and cousins. Even if the ones they are close to aren't on your side, be there to encourage the child's relationship. There isn't any reason why the children should feel as though they need to lie, sneak or disobey a parent just so they can continue these meaningful relationships.
As challenging as it might be, there is a good chance that your children will want other family members present at special events like birthdays, school functions and church activities. Come up with a set of rules for these times, including notification methods and follow those as these activities come up.
Your children should feel free to attend important events with extended family. This might include going to a cousin's wedding or high school graduation. Just decide what's important and be willing to compromise to make it possible for the kids to participate, even if they occur on your weekend, and the event is your ex's family member's function.
It might help if you put some terms into the parenting plan regarding extended family members. This could include behavior standards, notification methods and similar points. The more detailed the terms, the better things might turn out.