How you speak to a baby is more important than the words themselves, for sure. But at the same time, words do carry power, they have their own "vibration," so to speak. Babies understand a lot more than we think they do; before they can begin to speak they understand what words mean. I suggest that you respect your sister's wishes regardless of whether you agree with the reason. This lets both your sister and her baby know that you love and respect them. Also, let yourself relax and slow down when the baby is fussing, and allow the baby to fuss without being chastised in any way. Sometimes babies need to fuss and cry to release stress and tension, after all, they can't go jogging! They don't do it to annoy you, they have the same feelings we do, they feel stressed out sometimes and need somebody to listen to them "talk" or vent stress and tension or over-stimulation sometimes, and if they can do that, they will sleep more deeply later. Listening respectfully and kindly to their communication is the best way to show them you care about them, and babies who feel they are "heard" and respected tend to fuss and cry less.