The dog was the key!

*Spoilers ahead*

All along, since I saw the very alive twin sister, I kept saying "the dog!", bring the dog!
Was the sister of the deceased's husband blinded by her hate? She noticed on several occasions, the sudden dislike of the dog, barking at the fake sister. Everyone was surprised by it. With the two in a room, he'll go right to the right sister.

Didn't you wait for her, while in court saying, bring in the dog! The only only that can't be fooled by lies. Bring him out of the blue and you'very got the real killer!