I think your butt is sore because your babes are engaging and putting pressure on your cocyx area. I think it means the biscuit is almost done.
Try some pelvic rocks, cat/cow and some belly dancing kinda stuff, rotate your hips a lot, whatever feels good.
As for the sides of the hips, not the butt explicitly, well, everything is loose - similar pelvic rocking would help - and perhaps sleeping ON a pillow instead of directly on the bed would help, stuff one behind your back and one in your front, curl your leg over it - make sort of a v shape out of pillows so your pelvic basket has something even softer to lie in.
You could try sleeping on your back with your back proped up with pillows or a wedge - something like a 45 degree angle. This will take the pressure off your back, and off your hips. A pillow under your butt in this position might help, too. If you had enough pillows, you could sleep on your stomach w/a big cavety for your belly and perhaps boobs. Almost like the chriopractic bench for pregnancy.
And do those pelvic rocks, even while lying down.
hope that helps...