The way you ask this question, it sounds like a trick question. He cannot marry his widow's sister, because this means he is dead if his wife is a widow.
Is It Legal For A Man In California To Marry His Widow's Sister?
No it is not and sorry for your loss I guess you got the next best thing to the one you love enjoy your self who cares about the things people say keep it in the family thats bull what ever works for you congrats
Absolutely! Just be sure your not breaking any of Her family's laws. If their ok with it,, and sounds like you are, do it !