Like this but way more insane: Younger uncles and aunts (But I believe this sorta happened in House of the Dragon, prequel / spin-off of of Game of Thrones)
Let's say widow Viserys Tanaka had daughter Rhaenyra Tanaka and later remarries Alicent Hajime. If Rhaenyra is older (or even about the same age) than Alicent, then how might they address each other?
I have a feeling based on Spy x Family that the step-parent might address the step-child as 1st name-san Why does Yor call Anya as Anya-san instead of Anya-chan?
But I guess what would be more important is how far apart in age they are, their relationship prior to the wedding, etc over their legal relationship.