弟弟 denoting cousin (of younger age)?
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.