I think this is a big NO-NO. I would not date my sorority sister, blood sister, cousin or best friends ex. It's a matter of respect.
If she came to you and asked how you felt about the situation, that would make it a little easier to accept. Timing is another issue. Is the breakup recent? If you have been separated for a long period of time, I think I still would have approached you and told you I was interested him. That would have gaven you the opportunity to access the situation.
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