it's kind of hard when you are younger because they are your parents...hopefully in time they will see how responsible you are and appreciate that you are taking care of your little one. Just make sure you keep that baby safe. When I to my sisters house I taught my nephew to be gentle. Maybe you can take your little sister aside and show her, and teach her to be gentle. The best way to talk to your parents is when everything is cooled down, then you can tell them how you feel. (keeping it back won't help and hopefully by telling them calmly they will at least respect your wishes
)