He's the only brother you'll ever have.

[99bottles]

I found that sentence HERE.

Is it just me, or does it sound weird? Doesn't you'll ever have make it sound as if that guy is not your brother yet? All in all, is it natural?

 

[jutfrank]

Yes, it's just you. It's correct and very natural.

The reference is to the rest of your life. You have only one brother now, and you will have only one brother for the rest of your life. Remember that the verb have here is stative.