I know that our answers aren't what you're looking for but these thoughts all stem from our own personal experience. I don't think I've ever read a weight loss success story that started with "My family had an intervention..." or even "My family's subtle hints...".
There really is something that has to click within a person for them to take that step. And any amount of pushing from those close to that person almost undoubtably sends them deeper into either denial or depression about their weight. Wanting to help her is a noble, loving gesture. And yes, being overweight is a big deal that should be taken seriously...but it has to start with her.
The only thing I can suggest is demonstrating in your own life a very healthy attitude mainly through actions and not words. Make it about you, not her. Start a challenge here on 3FC, or sign up for a marathon. Buy a healthy (but tasty!) cookbook and make her a meal from it. Let her know you're shooting for your challenge or training for your marathon and ask her for help on decided a good menu for the week by taste testing some of the healthy recipes (again, shoot for things that are so tasty she wont believe they're good for you). Or have her spot you while lifting weights or go on a picnic outside somewhere that might lead to a short walk over to some landmark or vista. Again, the more about you it becomes, and the less focus that is put on her, the better. Keep it light, and keep it fun. The rest is up to her.
I hope that helps. I still strongly strongly agree that any intervention, no matter how subtle, would only result in damaging your relationship with her and pushing her even further into the path she is currently following. No matter how sincere your efforts. I really can't stress that enough.
Edit: I just read your response to Misti and I just want to say that I truly do not believe that we're trying to be defensive or present a bad attitude. We're honestly giving you pretty factual, honest, straightforward advice just as you have asked for. We're not giving you B.S. because our feelings have been hurt in the past or anything like that. We're telling you exactly how it will go down, even if it's not what you're looking for. We're all for support (we're one big family here on 3FC!

) but we're all here completely and utterly because we choose to be. And that, my friend, is what makes all the difference.
Best of luck for you and your sister...it's awesome that you care so much about her...definitely don't stop showing her as much love as you can muster!