Your sister is on a strict diet, but you are trying to persuade her to try one of your freshly baked cookies using ethos pathos or logos

You can try several different strategies. If you use logos, you can tell her that it is proven that a single cookie will not severely impact her progress as long as she maintains her diet afterwards. If you use ethos, you can tell her that you are particularly knowledgeable at diets or at baking, and try to convince her that you know best. Finally, if you use pathos, you can tell her how important it would be for you if she tried your cookies, and how happy that would make you.