As far as I know, sew is the only word in English that ends in -ew and is pronounced like -ow. All other such words are actually derivatives of the word sew, like resew, unsew, oversew, etc. If you think this is odd, wait until you get to though, tough, thorough, through, and cough!

Welcome to the strange world of English pronunciation. There is no doubt that pronunciation in English is confusing, there are so many contradictions which is why I would always recommend a knowledge of phonetics and also lessons specializing in pronunciation with a native speaker.:)