Gays who get along well with each other but (usually) do not have sex with each other often refer to each other as sisters.
The sister of a gay man is the (also) gay, but (mostly) platonic friend. Just like the best friend or the best buddy for straight people, this is a person who is considered the contact person for numerous questions and challenges in everyday life.
The designation "sister" alone shows here that it is definitely "more" than friendship, but family.
As is proper, with a sister (even if she is gay and is a man, not fucked! Everyday life shows that sex between friends (both straight and gay) is still common. Nevertheless, strictly speaking, the term "sister" can only be used in the context of platonic friendships.
Gay sisters thus interestingly always manage to overcome the prejudices of many other people outside the Scene to refute it. Unfortunately, there are still many people who think that gay men find it fundamentally more difficult not to have sex with other gay men around them. Yet honest sister relationships prove time and again that gays don't have to be unfaithful, constantly flirting people.
Many homosexual sister relationships are instead characterised by transparency and honesty. A typical sister is just as present when celebrating as when comforting. In short: it is incredibly good for many gay men to be able to exchange ideas with like-minded people and to trust their opinion.
By the way: some gays also call their straight male friends like to call you "sister". Mostly, however, this is an affectionate provocation, which the mate in question usually then knows how to take accordingly.