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We all know that Mom and Dad are capitalized when used in dialogue as a substitute for a name.
But is it the same for big sister, big brother, big sis, and big bro?
One example of the sentence would be this:
Big Sis? No. She gets another point.
Also, what if it include name like this:
Big Sis Tia? No. She gets another point.
I see a lot of novels using sister without any capitalization, but why? Because it’s a common noun?
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".
The Chicago Manual of Style says you should have "Grandmother", "Dad", and "Aunt Doris" but "my aunt Doris" - it says:
kinship names are lowercased unless they immediately precede a personal name or are used alone, in place of a personal name. Used in apposition, however, such names are lowercased.
In a case such as "my aunt Doris", "my aunt" is considered to be in apposition with "Doris", just like in a phrase such as "my favourite country Spain". (Commas are probably optional.)
You would also lowercase "I met his father", "a boy's father is the most important person in his life", etc. But use upper case in "I shot Father" or "I love you, Father".
In the examples given, you would capitalise "Big Sis" because it is used in place of a personal name in "Big Sis? No. She gets another point." and immediately precedes a personal name not in apposition (it is used as kind of a title) in "Big Sis Tia? No. She gets another point."
Having said that, this is only the Chicago Manual of Style, and other places may do it differently.
Source: Aunt-ing and uncle-ing, Grammarphobia, November 19, 2014
