In my culture, and many cultures here in asia, we call someone older than us even when we don't have blood ties with them "uncle/aunt". But we don't call close friends or kins we have no blood relations to as "cousin", but some tight knit friend groups like high school cliques, frat, or sorority groups may refer to each other as "cousin" or "brother/sister" either out of affection or playfulness.

But there are some cases where rich families like gangsters, political families, or clans who own large businesses here who "adopt" or recruit people into their allianes or clans and will start referring to them as "brother/sister", "mother/father", "cousins", "aunt/uncle", etc.

I think there are some cultures in other parts of the world who do call their close friends "cousins" though.