Word for a friend that is family to me

I am just trying to find a word that describes a friend who means as much to me as my family does. I have searched google but have not found such a word.

The words brother or sister can carry double-duty here, since they do literally mean a relative (I'll spare everyone a reference :-)), and can also mean

Slang. fellow; buddy

or

a female friend or protector regarded as a sister

If it's a male, you could say he's a brother from another mother. For females, you could go for the (much) less commonly used sister from a different mister. Both of these are very colloquial and rather light in tone.

You can use family itself.

He is my best friend. He is family to me.

OED definition of to be family:

to belong to a group of friends, colleagues, etc., who are close-knit, supportive, trusting, etc.

In a wedding toast I heard once I loved the phrase "family by affection".

I see you tagged this as single-word-request, but I don't think you are likely to find such a word in English.

Bosom buddy.

A bit vulgar, perhaps, but what's a little vulgarity between friends? I'm almost not joking.