So you would be cool with marrying your dead brothers wife?
That shyt is weird.
It's weird in our culture because it's not the norm but in other cultures a man would be obligated to marry his brothers wife if his brother died. It's a matter of perspective. If they follow up on their feelings and start a relationship they will face a mix of people with your reaction and people that don't really care. If the people closest to them don't care what others think doesn't matter.
As to whether it's wrong, a marriage is til death do you part. Once one party passes away the other is free to live the rest of their life how they see fit with whoever they choose. The fact that both of them are struggling with this shows they care/cared about the deceased a lot and respect the relationship that she had with the husband so I don't think it's wrong.