Why doesn't this paradox disprove (some) multiverse quantum gravitational theories?
The answer to your question for brane cosmology is simply that gravity follows the inverse-square law. We don't feel the gravitational pull of, say, Mars because it's so far away. Any other universes are so unimaginably (all right, that's hyperbole) far away in comparison that any gravitational effects by the matter in them would be very, very, very, tiny.

A similar argument might be to say that an infinite series always goes to infinity because you keep adding on terms. But this is often not the case, as with the series
$$\sum_{n=0}^{\infty}2^{-n}=2$$
It is infinite, but it converges.