I'm so sorry. It's hard to NOT be selfish in a situation like that. You have that side of your brain that tells you that you should be feeling happy and contented because it might be a better situation for them. But then you have that side of your brain of what you ACTUALLY feel.

Have you tried to talk with your sister? Not even to try to get her to change her mind, but just so that she knows how important her family is in your life. She'd probably appreciate hearing it and maybe it would mean she'd come back to visit more often if she knew how much it would mean to you.