This is especially difficult for surviving siblings when a child dies. There are some steps you can take to help you and your children cope and even find some joy in the holiday season. Often it's a relief to all family members just to say out loud that this is really a challenging time. Decide together what you want to repeat from past holiday rituals and what you would like to do differently this year. Note that some families decide to do everything differently, perhaps even going away to a new place that doesn't evoke so many memories.
Some families find comfort in doing what they've always done, and other families do some of both.
The important thing is that you and your family figure out what works best for you. Sometimes designating a special memorial time can free up the family for a deeper appreciation of the holiday and keep the sad feelings from being so pervasive. As you already are, remember to focus on the surviving family members, especially your other two children. They are still here to be cherished and celebrated. And of course they need special attention as they deal with their own loss.