meaning - "Even though none of you have yet to believe it" — grammatical?

We are powerful, even though none of you have yet to believe it.

Would be a perfectly sensible sentence for dialogue. The first clause states something, "we are powerful" and the second clause modifies that first one, by referring to why that clause may not be already evident.

That second clause "even though none of you have yet to believe it" is reasonable okay as a clause, though incomplete as a sentence.

It is a logically a double negative, and should strictly be "even though all of you have yet to believe it" or "even though none of you believe it yet". It's a reasonable double-negative in dialogue though; it's the sort of thing people would really say, and we know what they mean.

Even as given:

We are powerful. Even though none of you have yet to believe it.

It's reasonable enough, but the second clause being given as a separate (incomplete) sentence is reasonable as a bit of rhetorical license of making the clause stand more on its own and be considered more fully separate from the first. Or it could show it occurring to the speaking character after they'd already said the first bit.

So while you would normally want to avoid such an incomplete sentence in straight prose, you may deliberately use one in dialogue - your characters don't have to always speak grammatically perfect English - or as rhetorical license.