don’t know what to do with my sister anymore...

My mom is a single mother and due to our big age gap (I’m 25 and my sister is 13) and our family dynamic, I am functionally a second parent to my sister. I have just as much of a hand in raising her as my mom does, which my mom agrees with.My mom and I both have different ideas on how my sister should be raised, for example I don’t believe in spanking or hitting, but she seems to think it’s the only way my sister’s behavior will change. Over the past two years, my sister has become apathetic, disrespectful, and downright mean. She regular says things like “I’m the most beautiful person in the world,” and “I’m not going to grow up and do anything because there’s no point and I don’t care.” or “I don’t know why you’re talking to me because i’m not listening.” I’ve lost count of how many times she’s told me that she hates me, or that i’m lazy.When we try to talk to her and tell her that she’s wrong when she says these things, she says “Oh so you think i’m a terrible/lazy/mean/worthless person well i’ll just go live with my dad then since you hate me.” This usually sets my mom off and sends her into a rage which can end badly sometimes, with her “spanking” my sister with a belt.I’ve tried asking my mom to use alternative modes of punishment like grounding and etc which she does, but then the behavior continues and my mom uses that as proof that she needs to spank her more often. Arguments between them can get pretty nasty with some cruel words flying, and my sister isn’t afraid to cut a low blow with me either. Although when she says something hurtful to me I just take a breath, walk away, and come back later to administer punishment which is usually taking away electronics or extra chores. My mom was rough with me and I know how it feels, so I am gentle with her and careful with my words because I know that although she talks a big game, she’s sensitive to my words.My mom gives me just as much responsibility to my sister as a parent, but at the end of the day she is legally my mom’s child, and I can’t stop her from spanking her. I feel torn between them, because my sister’s lackadaisical and disrespectful behavior is uncalled for and should be punished, but I hate seeing her crying on the floor underneath my mom when she spanks her. She feels as if there are “sides” and that I’m always taking my mom’s side or that my mom is always taking my side.Any advice? via /r/Parenting https://ift.tt/3BfUQXm