We always have a problem putting dd(2)down when her step sister is here. I'm sure it's because she doesn't want to miss out. We took her to a New Years party and she wouldn't go down then - she saw the New Year with the rest of us and eventually collapsed in the travel cot!
DD is normally very good at going to sleep, but on nights when I think she just isn't going to I leave a side light on and tell her she can read her books. We often hear her moving around and talking to herself, but eventually it goes quiet and she drops off.