Tough situation

Hey, I'm Jana and I'm new to the forum!
I ended up here because I'd really like to hear your opinions on my situation.

I have a younger sister who is now 2 years and 8 months old. I'm her sister, but I'm home a lot with her too.
My mother has promised to keep an eye on my niece as well, who is now 2 years and 4 months old. My niece has been here since she was 6 months old.

Now, the problem is that my sister has a lot of trouble sharing her things.
When my niece comes here, things can get rough, and it's really like war between them.
My niece is really very insecure and cries over the slightest thing, while my sister is the complete opposite; she's very dominant, but I also think this is due to the whole situation.
My sister has had to share her things from the moment she was old enough to realize she was in this world, even sharing her mom and dad.
My niece's parents never bring any of her things from home, so she basically never has to share her own things with anyone else because she's an only child.
When we go to their place, most of the toys are usually put away, and my sister isn't allowed to touch much; we're not exactly welcome there...
So when my niece comes here and wants to take something to play with, my sister immediately starts yelling NO, and sometimes she even hits. My niece gets completely upset and starts crying very loudly.

I also notice that this situation is no longer healthy for either of them. My sister is very watchful all day long, keeping an eye on my niece. It's even so bad that she sometimes bites her nails and hands from nervousness.
I find this really hard to see because I think she also has the right to her own things.

I completely understand that she needs to learn to share, but I feel this should come from both sides, not just that my sister has to share all her things.
We've also asked my niece's parents many times to bring toys along, but then they bring them once and then they stay at home.
When they do bring something from home, my sister understands that it's hers, and then she lets my niece play with it and plays with her own toys.

My niece is also here full-time, 5 days a week, usually from 9 am in the morning until 7:30 pm in the evening, and then you notice that they just get tired of each other.
I don't know how we should solve this, but sometimes it's really unbearable, and they argue and fight so much that it's just no fun to watch them.

I also notice that my niece doesn't feel at home here. In the past, when they were younger, she was the one who hurt my sister; that has completely changed now.

I think this is no longer a healthy situation for both toddlers, because my niece's mother tells me that she gets very angry at home with them and doesn't listen, so I think this is just frustration because she can't be herself during the day with us.
We try to mediate and make everyone happy, but this sometimes just doesn't work.

Hopefully, you can give me an answer, and I'd really like to hear your opinions on the whole situation.
Greetings, Jana