Society is stringently designed around the nuclear family. This system makes it easy to treat families as economic units that are intrinsically tied together and emotionally bonded above all other bonds. Maintaining the nuclear family is difficult to do and often not beneficial for the members of said nucleus. Any child of parents who «stayed together for the kids» can tell you that they’d much rather their parents just got a divorce when they wanted one rather than maintaining appearances for outsiders while those inside the nuclear family were suffering. 

Your parents and siblings are not inherently more important than your aunts, uncles, cousins, grandparents, or any other member of your family. Legally, parental and sibling relationships are given more weight, but that is a social construct at the end of the day. Whether an uncle is a random man that you see once every 7 years or a surrogate father wholly depends on your family