Her "onii-chan" schtick is unbelievably annoying and she WILL lose. I mean, shirogane is in the mangas name...
Childhood friend syndrome. Usually means they rely too much on the past and don't act enough in the present. At worst they feel entitled to the MC because of their past, regardless of how insignificant it was for the MC. Relying on the past also means they rely on a time when the MC most likely didn't consider romance at all, so they place themselves in a box that doesn't contain romance, which naturally works against them.
Now, losing because her name isn't in the title isn't really something to hold against the character. However, since the real FMC had a decent date with the MC just last chapter and this childhood friend just failed at making any kind of move on her own means the race is pretty much set. And that's not counting the dual persona thing the FMC has, with both of them being attractive to the MC for different reasons. Shirogane's just better.