comment *contains spoilers*
kinda reminds me of this movie here:
http://www.imdb.com/title/tt0106808/
which I always heard was based off of a true story of sorts (maybe they changed some details for the movie).
I have to say the husband in this movie seemed very guilty to me. I was totally suspecting that his relationship with that woman was more than he admitted to and I wonder (if this movie was based on a true story) if the two of them did not plot the crime together. Maybe he wanted to get rid of his wife but keep his kid. I have to say it would have to be one of the more clever schemes I've ever heard of. Maybe he planned to go and be with his mistress and his baby after enough time had passed but then the sister got suspicious and found out about his mistress and so then he just pinned it all on her and acted like he had nothing to do with it.
I watch a lot of true crime stuff. Cops have said that when a pregnant woman is murdered that 99% of the time the baby's father is the person that did it.