Yes sad indeed. It's amazing what a person can do when they throw a tantrum, and this is what this is. A horrible deadly tantrum. If she could take it back, you can bet she would.I don't know how long this had gone on, but her statement to police give us a clue. Darin took her sister home that night and took a little longer than Darlie thought was appropriate. Now, I don't really think that anything was going on, but what did Darlie think? How messed up and unconfident do you have to be to think your sister is doing your husband. Something wasn't right, and it didn't just occur to her. She obviously had given the idea some thought. It may have been what prompted the separation/divorce talk.
Something else to keep in mind is the wedding dress and photo album open to a page of her and the boys. It had blood splatter, but was found closed. She quite obviously was doing some emotional thinking.