She's messy (as all kids are, I understand) but if you ask her to pick up after herself she throws a fit. My step dad literally does nothing so either me or my mom end up having to clean the whole house multiple times a day because she wrecks it. 

Yesterday she stuffed toilet paper down the sink and flooded the bathroom and smeared body wash and shaving cream all over the walls and shower. (Not just the shower walls.. I'm talking the ACTUAL walls). And today she squirted the foaming hand soap on the floor. 

My mom won't say anything to her because she's only been married to my step dad for about a year and he's really.. Emotionally unstable (like one second he's happy and then the next he gets in a bad mood and he's yelling at her for asking him what's wrong) and he for some reason never disciplines her at all. I've tried saying stuff to her in the past, like to sit right in her chair, pick up her dirty clothes.. Little things like that and he gets ma about it because I'm not her parent.. So now if I tell her to do something she tells me no and walks off. 

I just don't know what to do. I know it's not my place to try and discipline and parent her.. But no one else will do it. He lets her do whatever she wants and he's creating a disrespectful, lazy child who's only going to become a useless, irresponsible adult.