My sister is Favored..?

Hello. I am a 13 year old girl in seventh grade. I have a sister and she is 15 and a sopre. Sometimes she can be untrustworthy. What I'm trying to say is... when her baby sitters, teachers, friends, coaches meet her, she really looks promising to what they need. But honestly. She's a straight up switch up. What I mean, is one second she is a straight a, nice, perfect, goody-girl. Then whenever my mom or her "guardian" leaves, she is a straight up ***. Honestly. My mom's fiance yelled at me to pick up my soccer stuff from the bathroom. And, i sprained my ankle at practice, so under my breath I said "this f****** hurts" she looks at my mom and says "Hannah just cussed mom!!!" IT'S AMAZING. Then I said, like you never cuss. She of course denied it. she does, with her friends and my cousins to look cool. It makes me angry because, I cuss when it's necessary not when I want to look popular. So, I like limped out of the room and slammed the baby gate. Which is when my mom was like "YOUR ******* GROUNDED" along with My sister and her friend laughing. I screamed back down to my mom "OKAY, PICK YOUR FAVORITE DAUGHTER AGAIN." She was now, screaming at the top of her lungs. But I wasn't listening. It makes me angry. Because automatically when she's not there. It's my fault for defending myself when my sister slaps me in the face, or calls me fat, or just ****... I get blamed for trying to defend myself, when my mom told me i SHOULD defend myself. WTF? i'm sorry, i keep writing, i'm just crying right now, and want answers why my mom thinks my sister is so ******* responsible, when she is Truly abusive.../: