I would stand in the hallway opposite to the toilet door that was opened with DD sitting on toilet and i would look and sound excited (for her sake) and say to her " Lets do this poo together " ... then we would count together to 3 and then i would say " Push, push push yeh push that poo out, come on poo out you come yehhhh " ... until it came out ! Sometimes i would seriously feel like a crap after it myself, lol :lol:... It was like going to battle in some war but was so worth it. Only i would have to mix up the ' sayings ' sometimes to keep the excitement ;)
Also those rewards charts worked a treat, i would put a smiley face sticker for poo's and a star sticker for wee's (we would always put the sticker on the chart straight after the loo event so it's fresh in her mind) and when she got 10 stickers of either she would get a little gift, i would store in the cupboard gifts wrapped in cheap tissue paper, no gift ever over $2 ... sometimes i think she was more excited about the accomplishment of getting 10 stickers in a row than the actual gift.
Good-luck with TT for your sister's sake, it's not always as easy as we think until we get there but gladly the pattern does change and out of the blue all can come good out of it when you least expect it too :D