Not all of us have a support structure, and many of us just have top be self-sufficient, either as single people or as small families without the support of a convenient and nearby larger family. That's not how things used to be, but its how they are now.
It sounds as though your sister is exploiting your parents, expecting them to be true parents to her own child, while she parties like a woman without children. I'd guess that your parents talk a lot about thesister's child because he is very much part of their lives, and they don't have much else to talk about.
You don't know how your sister\s life is for her, maybe more disappointing than it seems to you - we really don't know how it is for other people. Truly happy people don't need to keep partying, and they don't neglect their children.
Milla's other suggestions make a whole lot of good sense, and are worth exploring.
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