In the first case, according to age. Generally speaking, if the age difference is greater than 10 years old (12 years old), you can address each other by elders, which actually means respecting others. In fact, it is a general principle not to be older than twelve years old. Of course, if the other person is reasonable, he will often say that we are of the same generation, just call me brother (sister).

In the second case, according to social relations. For example, A B and A A's father-in-law C used to be colleagues, and now A has joined this unit. Although the age difference between A and B is not very big, it is generally inappropriate for A and C to be brothers because A and C have had an affair, so A should call B an elder on most occasions. If Party A and Party B call each other brothers again, Party C will be very unhappy.

In the third case, it depends on blood relationship. For example, A and B are similar in age, but A and A's grandfathers are cousins, so A should generally be called Cousin B, not Brother.

The fourth case, a special case. Generally speaking, among strangers, young and middle-aged women don't like to be addressed as elders (except those with big age difference), while men over middle age like to be addressed as elders. Therefore, in unfamiliar situations, addressing female elders and male counterparts sometimes causes the other party to be unhappy.

Without the above factors, it depends on age. Generally speaking, a difference of 20 years is the right uncle and aunt. The difference is less than 5 years old, so it doesn't matter. Pay attention to the gap between the ages of 5 and 20. Of course, this is also a vague concept for reference, and we can't treat math problems with an attitude? Care about years? In addition, the feelings of the called party should be properly considered according to the occasion. Otherwise, it will make a joke. That's my answer.