Need Advice on Navigating a Challenging Family Situation

[TeenagersGuardian]

Hello everyone,
I hope this post finds you well. I'm reaching out to this community for some advice and support as I find myself in a rather complex and emotionally challenging family situation.
A little background: I'm in my 40s, childless, and have recently taken on the responsibility of caring for my 15-year-old niece. Her mother, who is my sister, struggles with severe mental health issues and is unable to continue supporting her. Due to the geographical distance between us for the past 10 years, I made the decision to allow my niece a certain level of freedom in my home while I worked on building a relationship with her.
Initially, things seemed to be going well, but in the past couple of weeks, the situation has taken a turn for the worse. I also share my home with my niece's 23-year-old sister and her two-year-old child. The older sister contributes a small amount to the rent and other bills.
The issue is that the dynamics in the house have deteriorated significantly. The teenagers fight like, well, teenagers, and it's been escalating to a point where I'm finding it extremely challenging to cope. To add to the complexity, the older sister has presented me with an ultimatum: either my 15-year-old niece leaves, or she will.
This has put me in an impossible position. Financially, I cannot afford the extra expenses if the older sister leaves. At the same time, I want to create a harmonious environment for everyone in the house. I also suffer from depression myself, and the stress of playing the role of a parent to a 15-year-old, combined with the older sister's behavior, is taking a toll on my mental health.
I'm seeking advice on how to navigate this situation. Have any of you been in a similar position or can offer some guidance on how to address the conflicts between the siblings and manage the overall dynamics in the household? I want what's best for my family, but it feels like I'm at a crossroads.
Thank you in advance for your insights and support.
Best regards,
TeenagersGuardian (Jolee)

If I’ve understood right you need to choose between the 15 year old or the 23 year old leaving which seems like a simple decision. The older one is an adult with a child and there’s no reason she can’t live independently. Whereas the 15 year old is a child and needs looking after.
However to avoid this I’d try to have a “family meeting”; work out what the triggers are and how you can all get around them, have some rules about walking away from arguments, the older one not trying to parent the younger one etc. Really I’d expect better from the older one to be honest and maybe you need to speak to her about acting like an adult.
Also are you claiming universal credit and child benefit for the 15 year old?
I would speak to social services; an early help team may be able to help with this ad surely otherwise the child will go into care and they want to avoid this

The 'teenagers' don't fight because one of them is an adult.

The child (15 year old) needs to be the priority. If the 23 year old feels the need to leave, that is of course up to her and I'd wish her well. The issue therefore becomes a financial one and I think you need to check that you're getting all the financial help you need. But as PP said, is there any way that issues can be addressed so that everyone ia more mutually respectful?

Sorry
But 23 is not a teenager ,I had a home and a child and was independent by then .the 23 year old needs to leave ,if that's how she feels ,she can't lay down the law in your home
There is only one teenager who needs protecting
You could claim benefits for having her ,there has got to be some benefits if your household income drops .
I'd personally get social services involved for support and look at benefits again.
But I'd also have a another go at trying to get both girls to play nicely for the sake of the 2 year old

Sorry ,meant to say
You are a wonderful person for taking in all 3 girls , absolutely wonderful