We've seen many changes when comic books get adapted for the big screen. Sometimes it's race such as when Michael Clarke Duncan was cast as Kingpin in Daredevil. You could even point out a red-headed Amy Adams cast as the traditionally brunette Lois Lane in Man of Steel. Now it looks like Lois' hair color isn't the only change that we might see in the film.
Where this comes from is on IMDB, Rebecca Buller is listed as playing Jenny Olsen. It could be easy to dismiss this as simply the possibility that we'll see Jimmy's sister on screen but there is an absence of an actor listed as playing Jimmy. There is also a scene in the trailer where Perry White is running from destruction along with Buller.
Watching the scene again, she's not carrying a camera around her neck but instead what appears to be an office badge of some sort. It's possible it could be a press badge. Why would Jimmy's sister be running from chaos with Perry unless she was either visiting the Daily Planet or working there.
Digital Spy also reports that Buller's role hasn't been confirmed by Warner Bros. and we know IMDB sometimes contains errors, especially in early production.
Does Jimmy Olsen's gender matter? It would add a possible different dynamic to being "Superman's Pal" and could also be competition for Lois. All we can do is wait to see what develops from this.