Dude, let it go. Making a big deal out of it will probably just come off as a "Let's Make It All About Me" moment, even if that's the complete opposite of what you intend. You said you "foolishly" had been "clinging" to this idealized version of your relationship---I think you kinda already know that you, as her sibling, need to let her go. After all, she's a well-rounded, functioning adult who has gone out into the world to live her life. That's what you WANT her to do. Be happy for and try not to dwell on the fact that you might not be as close as you want right now.
Besides, relationships change, people change, circumstances change. Such is life, my friend.
I think she's got it right here. Relationships do change. I'm not sure how many siblings you have, but I have two and the dynamics have changed so much over time. I remember being in school and my brother and sister being a lot closer to each other than to me. Then my sister and I were inseparable, nowadays I have a lot in common with my brother, etc.
It's also okay to need someone more than they need you. Just watch how you act on that and let it shape you. If it prevents you from doing more with your life, that's not healthy. But not all relationships are always going to be evenly balanced. I've been way more dependent on my family and friends in the past several months since I filed for divorce. I'll want to tell someone something, or just be lonely, and before it all fell to my husband. Now I am totally the one calling other people and bugging them. Do they think of me as often as I think of them? Probably not. But I know that they love me and support me, and hopefully I'm not going overboard with neediness.
Over-dependency is when you're so dependent that it's preventing you from having a happier, or more fulfilling life. I don't think you need to address any of this with her. Just sort yourself out, shuffle some priorities around, and try to have fulfilling relationships with the people in your life, whatever the terms are.