help me!!!

Crosspost "My mom [56/F] is angry with my niece [13/F] and doesnt want her around anymore. What do I [25/F] do?" from /r/relationships:

My brother's daughter (my niece, 13/F) came to my parents for a visit recently. To give some backstory, my brother is on drugs and is pretty much homeless so we dont see him anymore. My mom likes to pretend to the kids that everything is alright (she hides his drug use from them) but my dad is a more honest person. When my niece was visiting, my dad straight up told her that her dad is on drugs and to not end up like him. My mom was furious with my dad about saying this. Long story short, my niece doesnt really like her dad at the moment anymore because of what my dad said to her, and this makes my mom mad (my mom always defends my brother even though hes on drugs and doing nothing with life). According to my mom, when I wasnt home, my niece was "back talking her" (my niece is the sweetest kid ever, its really hard to imagine her saying one bad word to anybody, let alone my mom). So what happens is my mom got into a big childlike argument with my niece, and then ignored her for an entire day. Then that night, while drunk, my mom was all super friendly with my niece, laughing with her, etc. But the next morning, my mom was giving my niece the cold shoulder, and not saying one word to her the entire day. Then that night, my mom gets drunk, and says a lot of mean things to my niece, bringing up past problems that my niece has done, and my niece cried. Overall, my mom doesnt want my niece to come back again, unless its for a big family get together when everyone else happens to be there as well. It really angers me (and my dad) that my mom is acting like this. I told my mom that she needs to be the adult and try to make things better with my niece, but I dont think she knows what to say or where to go from here. What can my mom say to my niece to make it up to her? (Im not sure that my mom will even try to make nice with my niece, but at least me getting everyones opinions on here will give me an idea of what to say to my mom to try to push her into making up with my niece again). What do you guys think?

tl;dr My mom [56/F] is holding a childlike grudge against my niece [13/F] because she says that my niece was "back talking" her. My niece is a really sweet girl and its hard to imagine her saying one bad word to anybody. My mom doesnt want my niece over anymore unless its for big family get together when all of the other kids are over as well. What can I say to convince my mom to patch things up with my niece? What can I tell my mom to say to try to make things good with my niece again?