Non-Breath of the Wild Zelda theory.
I think that Link and Zelda, after Skyward Sword and on, are related to each other in some way. Not in all of the games because reincarnation is a thing but in some of them.
Taking Skyward Sword out of it because they clearly are not in that one and its the first in the timeline, I'm gonna start with Ocarina.
I think in Ocarina Link and Zelda are long lost brother and sister in that one. They only mention one parent for each of them. Links mom and Zeldas dad. I think during the Hylian Civil War they got separated from each other and Link and him mom were assumed dead. Meanwhile Zelda stays with the King and Link is raised as a Kokiri not knowing he's a Hylian until later. They also look very much alike. Both having blond hair and blue eyes.
In Twilight Princess I think they are still related in some way, maybe as distant cousins or something. There's not as much evidence in this one though. However the Heros Shade was confirmed to be Ocarina Link and he say that this Link is in his bloodline.
Wind Waker they are not related because the Hero never came back. Which means that this Link was reincarnated and not related to Zelda.