Not quite so and this has nothing whatever to do with Western practice. Some of us here are quite well acquanted with Eastern practices, and specifically here with Korean.

1) Nobody was considered in the line of succession, unless they were formally recognised by their fathers.

2) the children of concubines were usually either adopted by legitimate or full wives, or the concubines themselves were formally recognised, granted status and titles. One has to remember that there always was a difference between having a child by a concubine, a formal recognised status, and just having a child by any woman.

If these things didn't happen, the children had no rights of succession.

:since the
: japanese declared Yi Kang's 3rd child and
: down, illegitimate, these children were
: adopted under the collateral branches of the
: Royal Family under the line of prince
: Nahmyon down to Prince Jun of Unhyeongung
: Palace.

That still doesn't explain why his consort did not formally adopt any of these children.

: Only three children of Prince Yi
: Kang lived at some point in Sadong Palace:
: his first and second sons and the Prince's
: fifth daughter Princess Yi
: Haegyong.....................The right to
: sucession would not be this messy if only
: Crown Prince Yi Ku designated an heir
: decades ago.......
:
Yi Seok started his little campaign long before Yi Ku died. His claims were partly based on racist grounds. So now that his sister has trumped him, sounds like well deserved comeuppance to me.