try a star chart with the promise of having a 'mummy time' when she gets five stars .
explain to her that baby is going to sleep, and if she does not wake him up or whine for one hour she will get one star on her chart and when she has five you will spend .....minutes reading a story to her or playing with her. make sure you do not dictate the activity, do play like a friend not a mummy or teacher. once she has got the hang of the stars then increase how many she has to get before she gets a reward. if she wakes him or whines simply say oh well no star today. often just the threat of no star will stop a whine. the first few times when she gets a star go over the top with praise, ring granny, show daddy the chart together ... whatever just really make her feel proud of her achievement.