How to Handle Holiday Arrangements?

Two words... Open House. Don't do a sit-down dinner, but invite people to stop in and visit from 4-6. Serve cocktail foods, drinks, sparkling beverages and mini desserts. "Meals" require people to sit and talk... "stop in and say hello" implies that if it gets uncomfortable it's okay to leave.

Specifically invite your brothers to bring their girlfriends- it will continue to be uncomfortable until you meet them. Again- dinner is intimidating, a snack and a glass of wine is not.

If you don't like to entertain, then make it as simple as you can on yourself. Have people bring a dish to share and use paper goods!

I can completely sympathize with your situation. I think instead of making YOURSELF miserable, because you seem to be the one that truly puts the effort and work into making this happen for your family, just make arrangements to go out to eat. Unless your parents' divorce was really nasty, surely there are enough people to put one at one end of the table and the other at another end? I also like the previous suggestion of an open house- people can come and go as these please and its less pressure on you. Maybe a few years of this will make others appreciate what you are doing and step up a bit to help you!

If you can't do it all together, then have it on separate days. You can do one the week before the holiday and the other the day before the holiday. Alternate holidays. So if Moms family is the week before Easter, then the next holiday, they get the day before the holiday. They you have the holiday to either spend alone with your family, or with your husbands.
Make the dinner a potluck. A few weeks before the holiday call or send out emails and ask everyone what they would like to bring. Personally, I think it's up to Mom and Dad to provide the meat, but that's just me. When we have things at my house, my mom always pays me for the meat. If your brothers and sisters are young and not able to bring dishes, then the parent should bring enough for everyone.
As far as the kids not eating...well..I don't really think there's a solution for that. Just give them a small portion and let it go. They're distracted by everyone in the house and they just want to play. Chances are they'll eat once everyone has left.
My stepmom got all the kids some plastic sports type drink bottles from the Dollar Store. She put labels on them and each kid (there's 14 of them) has their own cup. If the cup is left laying around, we grab that kid and tell them they need to pick up their cup.
I think you should request that there's no alcohol at your home. After all, it's YOUR home.
Perhaps you could set a specific time. Say "We're getting together from 11:00 to 3:00." They you'd have a specific time and it might be that your brothers would stay the whole time.
Good luck! Entertaining two families is never easy.

I have one event. Our parents are adults, they should act like it.

My grandma always hosted our family get togethers for our convoluted split-apart, remarried, and generally crazy family. She would do more of a pot-luck, so that she wouldn't be stuck spending her entire day preparing. She would do the main course, chips and dip (family tradition dip.) and some meat/fruit/veggie trays. Everyone else would bring a desert, salad, appetizer, whatever. If the kids don't eat, that is the parent's problem. lol. She was very strict that if you want to drink something you stay outside or in the kitchen. Anywhere else in her house, it's water only. Alcohol included. We would always invite girlfriens/boyfriends, whoever. We always saw it as a great way to introduce the family! It was never awkward for us, because we always had so much going on that they would be incorporated right in. We usually spend the time before dinner hanging out and catching up, just socializing. Dinner itself is always very informal, because my grandma's kitchen was nowhere NEAR big enough to seat everyone. Heck, it was barely big enough to set out all the food! Usually we would serve ourselves, the adults would find space on a table or in the living room with a TV tray, and the kids (and teens + younger adults) would find room on the floor. After dinner, the designated unluckys get to start the dishes (For YEARS it was my cousin and I, until the next generation was old enough) while everyone who felt like it would help clean up the rest of the kitchen. We used paper and plastic too, but the pots and pans always needed done. That way when we left my grandma wasn't stuck with a trashed house. Then some of us would play cards, some would watch TV, some would head outside to do whatever... after a little while we would eat dessert. Then we would hang out for a little while longer, and people would leave. Usually, most of the family left at once, and my cousin, my other cousin and her kids, and myself would stay and hang out longer.

The only holidays she does are Christmas and Thanksgiving. Usually the other holidays were a come if you want to, and we will have a barbecue type situations. lol.

I'm not sure how many people are involved in your situation, or if your family would consider it rude to help each other out (or expect the help) the way we worked it... but it worked (Heck, still works) well for us. :)

As for you hosting your family, you'll probably have to keep doing that.

I always give 5 hour windows. Like another post said - Open house. My window is usually 11a - 4p. People are welcome to come and go as needed in that 5 hours. If they miss something, they miss it.

Instead of having a sit down dinner, invite your family for brunch. Talk to your parents and remind them they are adults and and should be able to sit down and eat a meal together 2-3 times a year. You can also invite your hubby's family, one big get toether for everyone. Or you can tell them to come at 8-9 AM for breakfast and that you are planning to go his family's in the early afternoon. At Christmas you can have a big gift opening at your house for everyone.
Have everyone bring something, pancakes and french toast can be re-heated in the microwave, coffee cakes, sweet rolls and breakfast sausage or bacon can go in the oven to be heated, then all you need to do is make a bunch of scrambled eggs and toast.
This leaves the rest of the day for your brothers to visit their girlfriends and your dad and his wife can go to her family.
My daughter used to do this and I would go home and relax, toss in a load of laundry and watch a movie. It also made it a day off for me and I could relax and be ready to face work again the day after.
No arguing or rude behavior allowed, make that clear. If someone is going to be unpleasent they don't get an invite.