弟弟 denoting cousin (of younger age)? 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.