I think the idea has promise, but would be very tricky to implement.
I’d keep part of Obi-wan’s ghost scene, so that it’s established that Luke has a twin sister. He simply doesn’t think that Leia fits the bill.
Here’s a potential ending:
yaddah yaddah yaddah
This would work really well as a post-credit scene.
Unfortunately, I don’t know if there’s a way for TFA to work without the knowledge that Leia is his sister, since Kylo’s entire being is built upon the assumption that Vader is his grandfather.
Maybe Luke is so close to Han and Leia that he calls them his brother and sister, so their kid is in a way Vader’s grandson?
I call my mom’s best friend my aunt, and her mom my grandmother.
Not sure how you could implement it, though.