The rule is very simple, and my own last name follows it.
In German, when you have two sequential vowels (EI or IE) the SECOND one is pronounced.
Anyone who is going against that (including Oscar Hammerstein, apparently) is incorrect. That would not be the original pronunciation of his name. It would be Hammer-STINE. Not STEEN.
Pretty easy to remember. Just pronounce the second vowel. If they've changed it somewhere along the way, that's their problem.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22