It always seemed to me that twist was meant to cash-in with Vader’s own revelation, and on its downside it shrunk that universe further. However, it’s not enough to ruin the movie for me, as the payoff to that sisterhood is Luke’s brief taste of the Dark Side, which is one the better scenes in the trilogy. The progression of the Skywalker/Vader narrative, while bumpy still managed to provide a satisfiying whole despite its improvised nature.
It feels to me like Lucas was tired, didn’t want to do any more Star Wars movies and having Leia be the sister wrapped up the triangle with Han easily
Having Leia and Han fall in love in ESB wrapped up that triangle already.