Want Baby Sister, Don't Want Baby Brother

The good news is if everything goes well, I should be expecting my second kid in July. I have to do my amnio test on Monday and hope the results turn out to be well.
When I first discovered I am pregnant, we ask Lucas if he wants a sister or brother. We don't think and still don't think he knows what is sister and brother, or boy and girl. The first time, he said 'brother'. Then it was forgotten for a while. In the last few days, he will out of the blue say 'Mummy, want baby sister, don't want baby brother'. When I asked him if he knows what is sister and brother, he kept quiet. But funny thing is he will said the same sentence now and then.