H may have wanted sisters to walk, but with no success. Or maybe now that he is less focused on OW, he is free to see this graduation opportunity as something that should not become a missed opportunity.
It is good that he discussed it with you, and included you. However it may end up meaning little more than he needed your help convincing her to walk. Doesn't mean it won't turn into something more, by surprise.
The best part is that she will have this experience and you will both be there to share it with her. That he and OW are currently breaking up does not mean they won't continue to bounce in and out of addiction to a hopeless R. Who knows? Be glad their drama is their problem. Be glad for your girl. Be good to yourself.
We're all thankful for the blessings that you receive and hope they continue to grow in number.
