Can 弟弟 denote a cousin (of younger age)? I was a little surprised by the following excerpts from 那个不为人知的故事 describing 杨锦天 both as 杨昭's 弟弟 and as the son of her 姑姑/paternal aunt (same surname（usually): 堂弟，paternal aunt：表弟，see comment #4 below):
来电话的是杨昭的弟弟杨锦天，。。。 其实这是个可怜的孩子。 三年前的一场事故，让他失去了双亲，杨昭的父母将杨锦天领回自己家抚养。杨昭也是那年回到了这座城市。 她在外很久，久得让她对姑姑一家的惨剧甚至不能感到痛苦。她难过，但是还不到痛苦的程度。至于这个弟弟，杨昭大他七岁，她与他的关系谈不上亲密。
It depends on writer story description, you cannot simply read it out of context.
来电话的是杨昭的
弟弟杨锦天 : denote sibling杨昭的父母将杨锦天领回自己家抚养 ：indicate he is not sibling, but cousins, but now adopted by 杨昭 parent.
至于这个
弟弟: denote 杨锦天 is treated as sibling instead of cousin.
This is not uncommon practice in other culture. Due to various unfortunate incidents(war, famine,natural disaster), it is quite common for human society to adapt sibling children as own children.
It is the adopting parent who decide whether to retain the relative title or not.
In such case, 杨昭 parent choose to make 杨锦天 as their own child. As in all cultures, family ties is very important. These small little details does matters.
Formally no. But it's common in casual spoken Chinese. Especially for the one-child policy generation, no one has any brother or sister, so we call our cousin brother/sister.
For your text, the little boy was adopted after his parents died. Even if he's a complete stranger before the adoption, he's now the woman's parents' child, ie her brother.
It's very complex.
"弟弟" means a lot.
The term '弟弟' only refers to 'younger brothers' (include half brothers or adopted brothers). It doesn't include 'younger male cousin' which has its own specific terms '表弟'(younger male cousins from 1. your mother's brother or sister [姨表]; 2. your father's sister [姑表]) and '堂弟'(younger male cousin from your father's brother)
*The same is applicable to '哥哥'. We do not address '表哥' or '堂兄' as '哥哥'.
The reason 杨昭 referring 杨锦天 as '弟弟' is clear from the excerpts
杨昭的父母将杨锦天(领)回自己家抚(养)
http://www.cantonese.sheik.co.uk/dictionary/words/38158/
领养: to adopt (a child); to foster (a child)
杨锦天 was adopted by 杨昭's parents. They raised him as their own son.
Even if 杨昭's parents were not blood related with 杨锦天, he would still be called 弟弟 by 杨锦天
mootmoot wrote:
It is the adopting parent who decides whether to retain the relative title or not.
It hits the mark. Apparently, 杨昭's father did not treat 杨锦天 as his nephew (sister's son). He adopted him to be his 養子 (adopted son)
Your adopted brother, blood related or not, is closer to you than your cousins do.
Side note from comments:
杨锦天 should had a different family name before he was adopted by his uncle from his mother's side. It was natural for him to adopt the 杨 family name from his uncle, who had became his adopted father.
