Turns out this might not be right.
聖 means saint. It’s the same word that’s used for real world saints and is gender neutral. Like in Japanese churches it would be used for St Patrick, St Mary, St Andrew, St Bernadette, whatever.
宮 is also used in real life, for Imperial Princes and Princesses. Again, gender neutral.
宮 was used for Charloss’ sister, who is the only female Celestial Dragon who has been named, but she’s also the only child Celestial Dragon that has been named. She was 15 pre-skip, when Charloss was 22.
It could be that 宮 is used for any Celestial Dragon Prince or Princess, being the younger ones, and when they turn 18 (or whatever age they are considered to be adult) they become 聖.
Basically, Sandman is jumping the gun because we don’t have enough named female or child Celestial Dragons. It’s based entirely on Shalria, and it’s just as likely (if not more likely, based on real-world usage) that Shalria has a different title because she’s a child and not because she’s a female.