I believe that Kathryn shouldn't be listed as David's (ex) wife in the family tree and here's why; Kathryn and David had false memories of getting married - just because they believed they were married, doesn't make it true. If she was listed as his fiancee, that would be different, because I think Abigail and Prince Charming were engaged. I think we should change it, so there's a small note underneath that says something like "David and Kathryn were believed to be married under the First Dark Curse".
Well, are character pages being merged or not? I noticed that Regina's is, but not anyone else's. If character pages are being merged then I think it'd be safe to just list Kathryn (or Abigail) as David's ex-fiancee since they were never truly married in Storybrooke but were engaged for a bit in the Enchanted Forest.
However, if pages aren't being merged then I suppose Lola987's suggestion works.