Hi Jessica,
I have 4 children and I have had a baby shower given to me for each one, I only had three showers, because my last pegnancy I was pregnant with twins, but it had been 5 years since I had a child, and I had 2 girls first and the twins are boys, so needless to say I needed boy things, but my point is, it depends on family and friends, whether they want to offer giving you a shower. I really don't think there is anything wrong with it. Especially since she is having a girl, and has a boy, she is going to need girly things!! Not only that, after a first child you can reuse some things, but somt things get too gross to use again, and look at how much things are changing from year to year.
Baby showers are so much fun too!!!! I had three showers for my 1st daughter and one of them was just a diaper shower, where the guests just brought diapers and wipes.
It may be proper etiquette but if think of it this way, if you got pregnant again and you were going to have a girl, wouldn't you LOVE to have a shower to welcome her into the world with family and friends?
I hope I helped.