I did sibling class for ODD when I was pregnant with DS and it was in mid December and DS was born the first week of February. I would do closer to due date.
We didn't do sibling class when i was pregnant With YDD because ODD already had been through it and DS was "too young" for the class. They say 3 and up. It turned out fine, he didn't miss a beat an is a great big brother. He loves babies though so that was a good thing