Thats not true. She was asked but didn't agree with some of the aspects of her part. So some things were rewritten and Terry Crawford played her part.
Can this be true? Millay who once played David's mother slated to play his wife in the sequel? This sounds a bit far-fetched, but considering the perennial absence of a mother during his formative years, maybe having David marry a maternal figure is not so unbelievable after all. A bit weird, though, to say the least! :-
But then again, this is Dark Shadows, after all!