So hippo birdies and all, Ray, but you make me feel old.
And speaking of old...
Possibly I'm missing a nuance of some kanji, but my understanding is that the present king of Sai is the aunt of the former king, and mother of his cousin 栄祝, the whichever minister. (The ministerial names are driving me mad BTW.) Japanese webpages agree that's the case. Well, 栄祝 is 29, so his mother would be in her 50s at most when she became an Immortal and stopped aging.
English Wikipedia would have her the former king's great aunt. Don't know if that's anime settei or someone misreading 叔母. Great-aunt would partially explain why she's presented as so old-looking in the anime, which otherwise I'd put down to Japanese artistic convention: all older people look 70, all six year olds are the height and shape of toddlers, and all ten year olds (great gargantuan beanpoles IRL) are drawn as stunted five year olds. (cough Taiki cough cough)
And Japanese webpage just spoiled me for what's going to happen in 華胥の幽夢. Ah well. I console me with that classic timewaster 12K TV Tropes.