Yikes that is a challenge! How many times a week are you doing this? I agree that some activities out of the house would be best! If they stay home, I would think you need a new activity every 1/2 hour. Here are some ideas:
* play with playdough -- make sure to have rolling pins, cookie cutters, etc.
* make tents and forts w/blankets and couch cushions
* have an early supper picnic ready to spread out and maybe one parent can give 2yo a bath while older kids watch a show/ do homework/ play video games
* have a handy supply of "new" DVDs (from library, swapped w/friends, or use "On Demand" if you have it) when quiet break necessary
* story time/ puzzles/ coloring
* buy a bunch of Christmas tree ornaments the kids can paint or color w/markers for them to hand out at Christmas
* become an AVID reader of Oriental Trading Company for project ideas ( I agree w/pp that Family Fun has awesome ideas too)
* make puppets out of paper bags and do puppet show
Some ideas for out of the house:
* send them to the park
* send them on a walk - create a scavenger hunt/ list of items to find, etc for them to do (in all your spare time!!).
* sign up 2 yo for class at Y/gymboree or some such place during that time and have grandparents and cousins watch
* buy membership to museum, indoor playground, etc and see if parents will take on outing
* send to McDonalds w/indoor play area -- grandparents can have a coffee, kids a yogurt parfait or apples and play away
I also think if your parents don't bring the cousins everytime, it may be less chaotic (are they watching cousins too?). I also think it is great you can work from home and have such great support in your family. Good luck.