Brother/Sister; Who are you?
As Sikhs we are a family, therefore we should see people who are similar ages to our family as such.
For example, if a female is around the same age as my sister would be - then she is my sister.
If a male is the same age as my dad, then he is my dad.
By looking at females/males in this way we do not have lustful thoughts. That's why its so important to bring this concept into our lives.
However, there are some people (possibly including myself) who mis-use this. If we are going to call each other brother/sister/mother etc then we cannot have lustful thoughts about the person, and we certainly cannot do anything that we would not do to our own blood.
This is very important, we must not kid ourselves into calling someone sister and then getting attached to her in an inappropriate way. This is why its best to only talk in groups of people. When we're put into 1-on-1 situations (online or in real life) we're alot more likely to say things that we wouldnt have said in the group.
The simplest way to find out if something is inappropriate, be it speech or conduct is to think if you'd do the same thing infront of Guru Granth Sahib Ji, surrounded by a group of people to your own sister/brother.
If the answer is 'No' or 'probably not' then we are doing wrong.