My girls are 8 and 10. They have regular sibling battles, so I feel your pain. My 10 year old knows just exactly how to push the 8 year old's buttons, in a quiet, sneaky, way, trying to play the innocent one. While the 8 year old tends to fall to pieces and over-react to her sister's actions. Like someone else said, I try to stay out of it, and don't take sides. The girls accuse each other of lying and me not believing them that their sister did something while I wasn't watching, but it really shortens the battles for me to not get involved.
I think it's hard for kids to come home from a long, fun day and go immediately into bedtime routine. They are overly tired, and the fun is over. You could try an incentive for getting ready for bed nicely. Sometimes we let the kids listen to books on CD on weekend nights in their beds, sometimes I pick up a kids magazine and save it for these kind of evenings, or save a treat for at home, only if/and after they get pj's on and don't starting fighting while racing for the bathroom, etc. I've really tried to stop making threats of taking away privileges in the heat of the bad behavior moments, because it just makes everyone more angry and tense. I've started letting them decide how to behave. Sometimes I calmly, in just a matter of fact, not sarcastic way, say "so, you're refusing to get your toothbrush" and walk away. A lot of times, that gives them pause, and when I don't get all authoritative and rush them, they start to do what is expected on their own. I admit, at the end of the day, and because I see the value of them getting enough sleep, and I am anxious for some quiet time, I hurry them through the bed time routine. Sometimes, it does go bad. I try to be patient in the moment, and it helps. The next day when everyone is calm, we talk about it. Sometimes there is a privilege taken away, or the answer to "can we . . . ?" is simply, "no, we didn't like your behavior before bed last night, let's see if it improves, and then we can . . . ." or then you can "have some TV or Wii time"