No romance or sex. Just something in which, despite their bad pasts (because at various times they've all been as bad as each other and they know it, though not necessarily to each other), even if they don't particularly like each other, China, Japan, and Korea are still family. Japan and China snark viciously at each other and Korea still has the fraternal equivalent of an Oedipus complex, but they do deep-down care about each other. Anon has a family-loyalty kink. I'm fairly open to situation, but I do quite like the idea of it somehow involving a tickle-fight (in which Japan loses pathetically) and/or verbally defending each other to either their bosses or other nations.
Bonus: Japan accidentally addresses China as "dad".