Yeah, that would be like revealing two already established seemingly unrelated characters are actually brother and sister! Who pulls that out in the third film and expects to get away with it? 😛
Leia being Luke’s sister wasn’t necessary to complete his arc and it was a weird path to take for sure, especially given their relationship in ESB.
Imagine Kylo and Rey being made siblings in IX. Nothing overtly sexual has happened between them, but they’ve shared moments that might be kind of creepy if you were to look back on them through that lens.
I still think the biggest mistake Lucasfilm made was knowing this would be a trilogy and not penning story treatments for all three films from the start. Instead of being a thoughtful and satisfying conclusion, IX might end up being hazmat-level cleanup.