Mom is a push over

So i have a serious issue with my mother. My mother allows my sister to walk all over her and I'm honestly getting fed up with it! My moms property is being used as a dumping ground for my sisters and her bfs crap! My sister has 3 non working vehicles on my mom's property, she has her bfs travel trailer, storage trailer, and wood pile out on her property! My sister also has some other random crap out here as well! My mother will literally complain to me about it but won't say crap to my sister bc she says she doesn't want to start anything with my sister.. It really makes me mad bc why tf should I be forced to listen to her complain about it but yet do nothing about it! This is how ridiculous my mom is.. I got a stocking for my mother to go along with mine and my kids at my place and my sister was coming over to drop something off while my mom was watching my kids and when I got home I noticed my mom's stocking was down and I asked her where it went and she said she took it down bc she didn't want my sister to see.. I looked at her crazy and was like are you serious right now!? And she said I told you I don't want to start nothing with your sister and you getting me a stocking and not her kids or herself might make her mad.. I was seriously thinking wtf is wrong with this family.. I don't know what to do but y'all I'm about to become a Royal demon and go into devil mode on my sister! I don't understand why my mom is so scared of her! My sister doesn't pay her bills or take care of her! I think it pisses me off so much bc my mother literally has no issue saying her mind to me though! Makes me feel like my sister is her favorite child or something. Now my uncle has his travel trailer out here and the other day she was again complaining to me about it even though last week she told him it was fine for him to put it out here! She lets everyone walk all over her! It is getting ridiculous!

The stocking was at my place! I don't live with my mother.