If you look in the early draft
For ROTJ script Luke was meant to keep his secret knowledge of Leia being his sister until the end of the film. Like Obi-wan told him
They are also wearing the same clothes as at the end of the film.
So I think it is better that it happens later. The only problem is.
“The force runs strong in my Family… I have it and my sister has it” but I thought this is the best handling I could come up with.