I very much doubt that you can help your sister.
Simply because you are her sister, anything you say will be taken amiss, because what we all want to say to a sister who can only talk about herself and her problems is "Oh, do stop going on about it. Take an interest in the rest of the world and not just yourself and your daughter."
We do not own our children, and right reason or none, your niece wants nothing to do with her mother right notw
Neither you nor your sister can change that, and your niece presumably does not want to.
So your choice is tough love -t elling your sister what I wrote above or simply telling her to stop moaning and get on with her life, or you hold your tongue and see and talk as little as possible to your sister, until she snaps out of her slough of self-pity.
This probably sounds harsh, but you really cannot make your sister change her behaviour.