Can You Date Your Ex-Lover's Brother Or Sister?

Can you go out with your [former] guy's brother or your [former] girl's sister?

What am really asking is:
- For the ladies, if you can date your [former] guy's brother.
- For you guys, if you can date your [former] girl's sister.

If "yes", what's your reason? If "no", what's your reason?
Sent from my BlackBerry® smartphone provided by Airtel Nigeria.