Dear CAB readers: Can a girl marry her late elder sister's fiance?

From a CAB reader

Can you marry your late elder sister's fiancé three years after her death? The late young lady was actually preparing for the wedding before she died in the car crash. Three years later, her younger sister marries her fiancé. and it got me asking myself plenty questions:

1) Is it too early? I mean, it's been three years.

2) Has he totally forgotten the deceased girl?

3) what if he hasn't, will the new bride bear to see him still thinking about her sister or mourning her on the anniversaries of her death and on her birthdays?

4) I wonder if the deceased girl is happy or sad about the situation? 

(You posted the story of the deceased girl when she died but please, keep it out to protect the innocent). I just feel it's unusual and I've been feeling uncomfortable about it. On the day of girl's burial, the young man actually promised never to leave their family. Little did I know this was what he meant. Sigh.