Well your niece is ALREADY displaying problems with this lifestyle and inconsistency and non-mothering in her life.
Now... your niece is 8. Just KNOW, that the "Tween" ages is from 9-12 years old. Preteens. This is also a crucial time.... of many developmental changes in a girl. Not to mention, the BIOLOGICAL changes, in a girl too. ie: body odor, getting their periods, changes in social influences/peers etc.
And, she will need to have someone to TALK to about these things and teach her and guide her about it, ALL.
Some girls for example: get their periods at 9 years old.
Google search "Tween Girl Development" and many good articles will come up. Read it. Arm yourself with knowledge about her age stage. So that you can help her in an informed, manner.
And since her Mom, has no idea, about being a Mother.
Maybe, you ALL need to sit down with your Sister and talk to her.
Because, her daughter is ALREADY, getting 'damaged' by it.
AND your Sister, NEEDS a Therapist. And taking her Anxiety meds. AND being RESPONSIBLE. Because, she is not, at all. She is behaving like a child, herself.
And she cannot parent.... competently.
And her judgement on things, is not in the best interest of her child.
Next: my friend is a single parent. And through the "Big Brothers,Big Sisters" organization, she got her son (a Teen) a "Big Brother." It was great and immensely helped her son. They provide mentoring and a ROLE model, for children.
Here is the link and maybe you can find one in your area:
http://www.bbbs.org/site/c.esJQK5PFJnH/b.1621293/k.BE24/H...
Because... your Niece, NEEDS NEEDS NEEDS, a good role model and someone who WILL, guide her and be someone she can talk to.
Since her Mom, is not that.
Again, your niece, is ALREADY suffering.
And it is best, to help her, BEFORE she is a Tween, and then a Teen.
Or she will have problems... and look for fulfilling herself, externally and with boys and outside the home. Which is really, not good.