On the capitalization of familial honorifics

Most English-speaking kids never address their parents or grandparents by their name, but only use Grandad/Gramps/Grandfather or something similar. So these monikers for parents and grandparents are capitalized because they're essentially being used as names.

Similarly, many kids would never think of addressing their aunt as Betty, but always say Aunt Betty. So we consider the "aunt" as part of her name and capitalize it.

For sisters and brothers, most English-speaking kids use names and only occasionally call them "sister" or "brother". If they always called a sibling by the name "Sister" or "Sister Susan", you would be justified in capitalizing it. But if they only use it occasionally, we treat it as a common noun. Similarly, most of the time, aunts, uncles, and grandparents call their grandchildren by their given names, so we don't capitalize "niece" or "grandson".