J.,
I really am saddened by stories like this one. I wish it was ONLY a story. Your sister sounds like mine, just very immature, she has no children yet, when and if she does have one, she said she will put them immediately into a daycare so she can still work full time. My heart breaks for these kids, I know all of us have different opinions about SAHM, and some moms CAN'T stay at home but want to. I know for a FACT children need their parents(or guardians), to grow healthy and happy lives. Children NEED love and nurturing, NOT a babysitter. Low self esteem, abandoment issues, etc, can come from this reckless behavior your sister is doing. Unfortunetly you can't tell her what to do, and she'll immediately snap into a great mother, but you can live by example, and if you can take care of your niece sometimes. I know it may be hard, but your niece needs family and love until her mother grows up. Children are so vulnerable to EVERYTHING, you or a grandparent being there will help her in the long run. And another important thing, never put down or yell at your sister in front of her daughter, she is still your nieces mother. That can strain their relationship even more. You already let your sister know how you felt, which is very good and I am sure it is noted, although not wanted.
I know you wish we had some special "advice" for you, but you being a great sister and Aunt is the best thing you can do now.
I will pray for your sister and niece.
If we could change people instantaneously, woman all over the world would stop nagging husbands, and their kids...where would we learn patience then??? (But if you do find a way LET ME KNOW =)!!!)
Good luck,
D.