In this lesson we have the sentence
Sarah’s Hochzeitsanzeige wurde vor zwei Wochen veröffentlicht. which means
Sarah's wedding notice was published two weeks ago.
In English, of course the apostrophe + s denotes possession, but I thought that in the German genitive you just add s (no apostrophe). Hence shouldn't it be
Sarahs Hochzeitsanzeige… ?
Is there a different rule for proper nouns?