we had lots of sleep problems with my DS over the summer. you can read my post if you like to get some ideas but we finally got it worked out. (DS previously was an excellent sleeper and napper.)
here are some ideas to think about it:
toddler clocks - eg. My Tot Clock (when clock is blue, it is bedtime adn child must stay in bed/room, etc). we use this now and it is pretty helpful.
Sleep Rules - re-establish sleep rules: some of our rules for example include 3 books, 1 hug & kiss, 1 drink of water before bed, 2 tuck-ins, stay in bed until clock turns yellow, etc. ie it is not play time, it is sleep time.
Sleep Chart/Book - they could have a little chart with pictures describing the rules
i personally would leave a gate up while re-establishing the sleep rules (ie stay in bed) esp so he doesn't bother his sister! oh no! if he wakes and wants to play and yells or whatever, they can say "it is time for bed" and leave. i would not interact with him much at all. give as little attention as possible. we had to put 2 gates up b/c DS would freak out and climb over the first gate! our situation was different but we did this routine. yes we were tired but after a couple of nights of "remember sleep rules" or "it is sleep time" and no interaction - ie, not tucking him in again, no more hugs/kisses - we did this outside the gate. (we were already tired from all the NWs anyway.) he started to give up. it got to the point where we would just say "no" and he would turn aroudn and go back in bed. then he started STTN again FINALLY. it was a miserable time and wish we had been sterner sooner. i was very depressed due to our sleep problems and regret not nipping it in the bud.
HTH!
