@mialinay if your asking if I'm 13, yes I am, and I told my mom that I don't like having that child over all the time and she called me a snotty selfish brat.

@mostlyghostly06
I really think you should try to get into family counseling with your mom. It sounds like this is an issue that might be too big for one person to handle. A bit of professional help can go a long way here :)

Have you asked your mum why she's looking after this girl? I know it may seem like a dumb obvious question but it would help to know her own motivation/ reasoning so we can figure out a compromise

Mom won’t want to go to counseling she prob just wants me to deal with it and she’s going to get mad again if I ask why but I do remember saying she misses having a sweet child.

*her saying

Hopefully this isn't too personal to ask but was your parents divorce a clean or messy one? And do you know who instigated it? @mostlyghostly06
I'm sorry you even have to deal with this, divorce is hard at any age but especially during your teen years. You are loved though and we're here to support you.

1) sounds to me like you're jealous of the little girl (that's not a dig or an accusation as you have every right to feel that way) as you feel she's usurping your position of daughter.
2) you're mum is acting like a bit of a dickhole tbh, I know I know she's going through a LOT of pain right now but that doesnt mean she should take it out on you :/
3) I can see why your Dad would have thought what he did, if you entrust someone with your child you have to have a very strong relationship with them, not necessarily romantic but still a close relationship which your Dad realised and may or may not have misinterpreted.

Your options are:
1) talk to your mum like Lihea suggested, although if your mum isn't willing I don't see how it'll help.
2) make the best of the situation, support your mum as best as you can and try to form a relationship with the girl, it's not her fault.
3) just get over it and get one on with it.

Perhaps it is not relevant to dissect Mostly's parents relationship as the problem concerns the direct relationship between Mostly and her mom.
It's clear your mom is acting out of an emotional need and not a rational one, as I said before. Therapy might help her deal with her needs in a less disruptive way, but if she's not willing to do that then there is only one option left. Mostly, you will have to find a way to bearably live with this situation.
While I could give you some advice, my experience is that these situations tend to shift and change. So what might be a workable bit of advice today might not be something you can work with tomorrow.
Is there a way for you to get some professional counseling yourself? It could be a huge relief to have someone trained in dealing with these kinds of situations think with you every week and try to help you navigate this tough path.
At this point I would advice you please think of your own mental state and find someone who can help you!

I'm sorry this is happening, I don't have any advice but I hope it works out okay soon

@parisqueen actually my parents divorce was filed by my mom and it wasn’t messy my dad signed the papers without a fight.

@mostlyghostly06 it must be a cold war situation at the end or they might not be including you in the arguments which is understandable as you are 13....how I imagine is your dad accused your mom (irrelevant of the accusation being true or not,tho I think it maybe true) and mum knowing no point in resisting ended everything and dad knows there's no point now so he signed them without any opposition..tho yeah 2 questions, does your mom feel irritated/ashamed while conversing with you?? And has your dad tried to get you through law??/ how are your relations with dad???
(All this is assumption cause of incomplete data so really sorry if I hurt your feeling and I really wish that everything gets sorted out quickly so you can have you peace with it and be happy...)

@blazingfrags ur not hurting my feelings at all my mom doesn’t seem irritated when she’s talking to just me but when the child is around she completely changes which is understandable the child is 3, and I see my dad every two weeks and in between I just stay with him for a weekend every two weeks so he has no reason to, and I have a pretty good relationship with my dad.

I'm glad it wasn't a messy divorce. If your mums relationship with this 3 year old is really effecting you, could you live with your Dad full time? Or would that be too difficult for both your parents?

Well my dad lives a city away but he’s moving, and I also have my dogs and my ferrets that I can’t leave behind.

And you can't bring your pets with you? Would you want to live with your Dad if that was an option anyway?

The pets may be more like Family pets, not hers alone to take

Yes my pets are mainly mine but my dad wouldn’t let me move them there because ferrets really smell and the cage is giant and I also have three dogs in which they do not want.

This is a really tough situation, have you told your dad about this? Maybe he can offer a good way of approaching the issue with your mum, since he and you knows her better then all of us do.

If you get asked to do any chores, just ask her to ask your replacement, use this as an excuse to chill whenever you want