Keeping it even with birthdays and holidays

I have 3 kids, my husband and I have 1 child together and one on the way and we came into our relationship each with a child from another relationship. His family favors his first born and my family favors my first born. It's creating alot of problems with our kids. How do we get our family's to keep it even? We keep having to repeat ourselves asking them to keep it even but most family members never do. Please help!