Okay, you CAN'T make everyone happy! They are grown-ups, and some of them sound pretty immature. You cannot lower yourself to their level - they are actually controlling you through their behavior.
You handle your parents, who are coming from far away, and deserve some special time with their grandchildren. Your husband can handle his parents (leaving you out of it) and tell them they are making HIM miserable (not you) and also screwing things up for the kids.
I know the frustration of dealing with a stepchild whom you don't see until later in the day. Maybe you could have the morning be "Santa's gifts" for your kids, but save the family gifts until Wyatt comes. That spreads it out over the day anyway, which is better for the kids. Skylar won't know what's going on, and Trevor is old enough to defer some of his gifts until the afternoon. Bring the in-laws over for the afternoon since Wyatt is their grandson, and have your husband tell them they are staying home in the morning because they are so unpleasant to you and your parents.
Everyone can come for Christmas dinner if they behave, otherwise go for the dessert at their house as the previous poster suggested. You and your husband have to establish ground rules for your home. No one can take advantage of you without your permission, so it's time to set boundaries and stick to them.
If your parents don't come every year, then your in-laws can come on other Christmas mornings. If your parents DO come every year, then both sets of parents need to suck it up and learn to behave for the sake of the grandchildren. Otherwise, they alternate years - your parents stay home one year and your in-laws get the Christmas morning deal that year. Your in-laws should be thrilled that they get to see the grandchildren all the time, and they should support your parents just being occasional visitors. Your parents should appreciate that the in-laws are there to help (if they do), and be able to enjoy the few visits they have.
They are not modeling good behavior for their grandchildren, who need to grow up learning about the JOYS of family and not the silly jealous competition garbage. They will never be able to seek out healthy marriages for themselves if they haven't witnessed it - and you and your husband can tell both sets of parents that. The sooner the better.
Otherwise just do your own thing and make the rules for your own house.