Looking back I've never gotten back with an ex. But I have made every effort to make a relationship work while it was ongoing. There comes a point at which things cannot be resolved if either person is unwilling to grow.
Any of us as individuals can only ever be half of the interaction. Yes we have to own our crap and own our behavior and be honest with ourselves about what we are contributing to the interaction, but at the end of the day we remain only half of the interaction.
We have no way to make another person change their behavior. Ergo, if the other person's behavior is at issue you either tolerate it or leave. Those are truly the only choices you have. Now. If the other person assigns you enough value they will be mindful of how their behavior affects you and they will modify their behavior for the benefit of the relationship, that is the growth part of learning to be considerate of your partner, and it is a learned thing. If they don't? You have to respect yourself enough to walk away.
My bet
@Dr.Suave is that your sister reached a point where, to preserve her own self respect, the only choice was to leave. But she had to arrive at that conclusion on her own.
Humans are inherently selfish and yet humans also crave social contact and meaningful connection, but these things (selfish nature and desire for connection) are at odds and in conflict. It takes compromise to make it work in any relationship. It takes self sacrifice and consideration of the other person as well as action (active follow through) to get there. It takes a level of maturity to get there, and some people never attain that maturity level.
I am an extremely patient person in relationship. If I'm in, I'm ALL in. But I also have boundaries that are based in my own self respect. I'm also transparent about my boundaries and expectations. But there is still always and forever that dovetail of how I interrelate with my husband, and his personal growth trajectory is different than mine. Typically one individual has farther distance to travel than the other to reach harmony in relationship. Patience is requisite in allowing that growth and journey to occur.
And that is why marriage (for example) is difficult. Both people must want to accomodate each other, and make the effort, and at the same time be patient. Not easy. Worth it, but not easy.
