Having "no interest" in learning other languages is a serious bad mark against your brother, I'm afraid. He might be a "very good" programmer, but he'll never be great - never be a "hacker", by pg's definition.

I don't know any hacker who doesn't have an intense, irrepressible interest to learn new stuff - languages, frameworks, whatever. I also don't know a single hacker who hasn't learned well over half a dozen languages to a proficient level. And no, CSS and AJAX don't count (unless the Javascript is taken to a scary level). Sounds like your brother is a designer with some coding skills, not technical co-founder material.