What do many people mean by calling each other "in-laws" online?

Many people on the Internet call each other "in-laws", which means brothers and sisters who are very close.

In real life, after a man and a woman get married, parents call each other "in-laws". In some parts of China, the word "in-laws" is widely used, which means that parents of husband and wife call each other "in-laws". It also refers to the mutual name between michel platini and dopted mother, that is, herself and her children, and is called "in-laws".

Being keen on spoofing some internet terms, the new usage of this old word is often very popular.