Hello and welcome to the family! The dynamic sounds very stressful but at least they live several hours away and you don't have to deal with this daily. Or are you planning on staying longer due to your sister's sickness?
Also, what would your "more serious talk" look like? Talk that out a little bit so we can better understand your goals in that conversation.
With mental illness, your niece will only get help once she realizes that she needs it on her own. Nobody can force her or convince her of anything, not even professionals. She has to actually want to change and heal before signifigant progress can be made.
While it would be great to get your sister's house cleaned out and organized, it might not be possible. And that's okay. Do what you can, don't argue about any of it, and they'll decide what and how to maintain the home.
I'm so sorry about your sister as well, I will say a prayer for her now.


