My only issue with this is that the anime has shown that she had her doubts. Now, doing what she thought was right even at the cost of her own comfort is admirable, so I'm not saying that Ranka made the wrong decision with the information she had. However, part of maturity is seeing things through, no matter how hard it is. It wasn't all bad for Ranka, there were things about fame that she enjoyed, but when the call came for her to step up and push aside heartbreak for the sake of her friends, she wasn't able to do it until Sheryl slapped some sense into her. Being a professional is more than getting satisfaction from protecting your loved ones or enjoying the benefits of fame, it means you often have to do your job when you least feel like it.
It's like Sheryl trying to drag herself out of bed on Galia 4 to sing for the Zentradi--she put the subduing the conflict above her own comfort and needs. Sheryl has been singing longer, and has more experience, so I don't think we're supposed to hate Ranka because she's not on the same level, but Ranka's weakness in this regard isn't something to be admired. The way it comes across is that she wants to make and eat the cake, but balks at doing the dishes.
To be fair, this is a Ranka discussion thread, right? My only goal is to explain why people might be frustrated with her characterization. I don't think it has anything to do with the love triangle, but how she's been written in these last few episodes.
People have mentioned Nausicaa. 2ch has also picked up on this, but I think Anonymous over there said it best that the difference between Nausicaa and Ranka is that when you watch Nausicaa, you get a sense of her love for both sides of the "conflict". She loves the insects, but she also loves her Valley, and would mourn the loss of it terribly. This is not to say Ranka doesn't love her family and friends, only that Nausicaa tends to look at both sides of the equation and balances herself much more carefully than Ranka does.
Ranka wanting answers is a step in personal growth. She's no longer a scared little girl, hiding from the pain her memories could bring. But at the same time, it is a personal choice, not an unselfish one. Just like confessing to Alto was a good choice for herself, but also puts a burden on Alto, as she leaves the fleet before he can even discuss anything with her.