If my memory serves me, the same issue was brought up by Bingley in referrence to a different word.

Ever since, upon seeing a word spelled with a "Y", I've repronounced it to make the "Y" a seperate, stressesd syllable... fun, but not productive.

The ile part of sterile is pronounced the same as in isle/aisle. Yep and nope... It depends upon meaning. In order to differentiate between *sterile meaning "clean" and *sterile meaning "unproductive", the later is given the "aisle" pronunciation (not like they would ever be confused)!