It's absolutely valid - although if you find yourself piling up too many relative clauses on top of each other, it's probably best to rewrite the sentence so that it doesn't get too wordy.
In general, the order of relative clauses does matter - the one closer to the main clause modifies the main clause, and the one separated from the main clause modifies both the main clause and the first relative clause. Consider the following two sentences:
Bill was the only one who took part in the robbery who got arrested.
Bill was the only one who got arrested who took part in the robbery.
In the first sentence, multiple people have taken part in the robbery, but out of them only Bill got arrested. In the second sentence, multiple people got arrested, but Bill was the only one who actually took part in the robbery.
In your example there's much less of a difference, as "out of all the people I know of, my cousin is the only one who speaks English well" and "out of all the people who speak English well, the only one I know of is my cousin" are more or less interchangeable statements. Still, depending on whether the context is "people you know of" or "people who speak English well", you may opt for one or the other.