I think they can. When i was pregnant with dd3, the big girls had their minds set on what they wanted. One wanted a boy and of course the other wanted a girl!
Anyhow, I spent a fair bit of time explaining to them that we didnt get a say in what gender the baby was. In preparing them for a boy, I kept on saying how it would be nice for dh to have another boy round the house, that a boy means they can look after them when they are bigger etc. Also having a brother means that they would have had a prince to play with when they were playing their princess games. My girls were never keen on looking at boy clothes, but I think that is because they couldnt relate to wanting to wear the boy clothes.
In the end though, it didnt matter whether they got a brother or sister. They got a cute little baby, who was theirs to love and be little mummies too. Any disappoint was quickly thrown out the window.
I wouldnt worry about it too much now, try and brace her for the prospect of having a baby brother. But I would be focusing on how she can help you and the baby when it arrives. What her role is going to be and how important she is going to be to her baby sibling. That may shift the focus enough that she wont mind what the gender is.