This is something commonly seen in various people, referring to someone of the
opposite sex. Usually it comes in the form of 'I consider so-and-so as my
spiritual brother/
sister' or 'But youre like a brother/sister to me!' And 99% of the time, it is a
lie.
People who say this mean one of two things:
'I don't want to go out with, have sex with, make out with, or in any way be romantically involved with that person'
or
'I want to fuck that person's brain out, i am madly in love with that person, but for whatever reason i can't admit it now.'
The first case is commony addressed to a single person who has made advances at the alleged sibling. It's right up there with 'let's just be friends' as a lame way of saying you don't want to go out with someone. In some cases, the person saying it may THINK they relate to the other person a s a sibling, but this is generally not the case. They will soon realize in this case that they just consider the other person as undesirable. In many cases, the person disappears forever after making this comment. 'Well, i guess you don't like your brother much if you ignore HIM like that too..'
The second case scenario is a commonly heard form of denial. It may be addressed to the person in question, or to mutual friends. It is usually a way of convincing one's self that they do not want this person, when in reality they do. Usually the person saying this has a reason to deny wanting the other person; they have a boyfriend/girlfriend, the person they want has a boyfriend/girlfriend, they consider the person they want out of their league or think dating that person would be socially unacceptable. Often, this 'sister/brother' stage lasts about a week and the people then rip each others clothes off and have at it with each other. Sometiems this stage may last much longer, but both parties generally know it to be a lie
The only cases i have ever experienced where one actually considers someone of the opposite sex a sibling are when they ARE a sibling, or sometimes when the person is of a differing sexual orientation (see finding out your friend is a lesbian)