My only thoughts are doing a real routine for bedtime.

Dinner, short play, bath, story, bed. If you have no other stimulation - i.e. the TVs not on, and the other kids aren't partying up! Even go to bed yourselves as Barb suggests. Then slowly bring it earlier and earlier. She's not going to want to stay up by herself - so if she sees the lights going out and everyone winding down, then this should help her understand.