When parents divorce, they typically understand that it's best for their children to continue to have a relationship with their grandparents on both sides of the family. They may even write grandparents into their parenting plan if they're going to be regular or frequent caregivers for the children.
Fewer parents give serious thought to their children's relationships with their cousins, aunts and uncles to whom they may be close. They typically assume each parent will arrange for their child to spend time with their side of the family during their parenting time.
Why the commitment has to be made by both parents
That might sound good in theory, but life isn't that simple. A cousin from your spouse's side of the family may have a birthday party on a weekend when you have your child. Your brother may have an extra ticket for a Cowboys game during your spouse's parenting time. Likely neither of you wants to deny your child these opportunities.
If both of you commit to letting your children continue to spend time with the family members they care about – regardless of how you and they feel about each other – it's typically healthier for your child. The less a child's relationships are disrupted by divorce, the bigger the support system they'll have.
Disparaging remarks can't be condoned
Of course, this assumes that these family members are good influences on your child – or at least not bad influences on them. Further, all family members need to keep any negative feelings about either you or your co-parent to themselves when they're with your child. Just as parents should never criticize nor demean their ex in front of their child, neither should any other family members.
While it can be frustrating to have to give up part of your parenting time with your child to let them do something with a former in-law, denying them a special occasion likely is only going to give you a very unhappy child. If you're generous with your time, your co-parent is more likely to reciprocate. If you believe that time with these family members needs to be addressed in your parenting plan, it may be worth modifying the plan so that everyone's working under the same expectations.