Should You Date a Friends Ex?

Kim in Mountain Home is a real predicament! What do you think she should do?

Oh boy! Kim in Mountain Home has got a problem. Today we received this letter.

Dear Rick & Carly, 

Nine months ago, my best friend died in a tragic motorcycle accident. Now her fiance and I have started dating. My brother says this is cheating and that I am breaking the unwritten rule that says you should never date someone your friend dated. My best friend disagrees with my brother. I'm confused. Am I doing anything wrong by dating my late friend's fiance? 

Kim 

What Should Kim Do?