Most Helpful Guy
Well, both guys and girls do this, and both tend to for the same reasons. People like the idea of being able to feel indifferent to something, it makes them feel powerful. So, when someone cares about something (such as the joy of being with someone) and there's something that prevents them from being able to have that something, they'll do whatever they can to make themselves stop caring about it. It's why people will try to pretend they don't care in an attempt to fool themselves (although people also often do this to make someone miss them), why they try to find something else to care about (like jumping into a new relationship), why they want to break contact after a break-up. They feel powerless, and they're scared of that feeling.
Your sister sounds like an admirable person though. She wanted to communicate, to move on by facing the truth rather than to hide from it. It'll benefit her in the end, don't worry. :)