It's all good to help someone out when they're in need; this is an essential part of being a family, friend, or even part of a community. However, there's a distinct difference between offering your assistance to someone who is beyond grateful for a helping hand and being forced to help out someone who feels entitled to your time and resources.
This isn't always cut and dry either; help can often start with a lot of gratitude and then become a routine expectation over time. Here, often, the best intentions are had, but the person receiving the help becomes reliant on something that was only supposed to be a temporary offering. Sometimes, though, that person you're helping—be it a family member or friend—is already of the entitled sort and views your assistance as an obligation rather than a gift.
This is the situation at hand with these two sisters, the older one being the "favorite" child of the family. The younger sister, who shared the story with this popular online community, explained how her sister expected her to help out with her two kids—despite not offering any care for the younger sister's own children. This came to a head and broke out into a family feud when invitations to a beach holiday weren't extended.