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.
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.
A bit vulgar, perhaps, but what's a little vulgarity between friends? I'm almost not joking.
There's no single word in English, at least not one that would be understood by the average speaker. I'm not going to say you couldn't dig up something archaic, but that wouldn't really help.
I would use the term
kith and kin.
Kin is "family" and and
kith would be "like family".
I tried searching for a proper definition and it turns out there's not much supporting this definition. It may be a regionalism or it might be archaic.
Soulmate, perhaps - a person who shares a deep understanding or bond with another; esp. one ideally suited to another as a lover or spouse.
1822 S. T. Coleridge Let. to Young Lady in Lett., Conversat., & Recoll. (1836) II. 89 You must have a Soul-mate as well as a House or a Yoke-mate.
1847 Howitt's Jrnl. 6 Mar. 128/2 She could be neither soul-mate nor help-mate to such a man.
2006 Psychologies (U.K. ed.) July 14/3 We all hope to make a few deep and lasting friendships, but... we can have other friends who are fun, kind or useful rather than being soulmates.