Ginny didn't know about the engagement, but she would indeed have been a bit upset had Hermione decided now was the moment to share her own news. It might have been selfish, but Ginny wanted this particular one for herself - she'd be happy for Hermione and Ron's news to come out tomorrow, even if she wasn't going to agree with the idea that they should get married right now.
"Well it....Harry and I know this is where we were going. We just got a little ahead and skipped some steps, but it doesn't change anything. We're still Harry and Ginny - we've just got a little boy coming along for the ride." she smiled, a picture of a black haired baby lodged so firmly in her mind that Ginny knew she'd be a tiny bit disappointed if the baby came out with red hair.