Thai Twitter users actually idealized The British Royal family lol. People who don’t touch grass dislike their own royal family because the incest. (There are people who dislike them for more normal political reason, but it is not important now.)
Basically, marrying cousin/sibling is the norm 70-80 years ago. And most of people do not have record of their own tangled family tree and they turns their attention to the royal families that has a record far back.
So, in Asia countries, being a sibling and being in that kind of relationship is nothing too strange. Even if it is now frowned upon and illegal in most places. This lead to young twitter Thai people screaming at old literature.
Basically, I bring it up because the nature of Keaya and Diluc may be weird (depend on how old the writer is), it is not uncommon to call your sworn brother’s parent your own parent and then bang your “sibling” the same night.
Sorry if I bring up “They might canonically engage in something like an incest relationship based on some old Chinese poem I read.” into “Twitter users mad at people shipping Diluc Keaya because they don’t understand the Chinese concept of sworn brothers.” table.