I do a lot of baby knitting and I've often wondered why on earth it matters which side the buttons are on. I can't see that it makes any difference, nor do I think anyone would be worried about it. Your sister would just be so happy you went to the trouble to knit it.
Its very cute, and would look just as nice for a boy, without the flower of course. I would have just a plain pocket, no motif on it at all. You needn't do blue, there are a lot of nice colours suitable for a boy, and you could experiment and find two colours that contrast nicely, or a lighter and darker shade of the same colour. Buttons are always the finishing touch which can make a baby cardigan look that extra bit special, so find some really nice ones that look perfect with the yarn you choose... probably match the buttons to the colour of the yarn you use for the edging.
Congratulations on being an aunt-to-be!