Here’s the gist: Three years ago, Elsa and Anna’s parents sailed off to a wedding (Flynn Ryder and Rapunzel’s wedding) and they died at sea. (Frozen came out almost exactly three years after Tangled did.) Their shipwreck is the one that Ariel explored in The Little Mermaid. It even makes sense geographically!
The king of Arrendelle is actually Rapunzel’s uncle and magic only affects blonde children in the family!
Baby Rapunzel and Elsa were both blonde. If Elsa’s dad was Rapunzel’s mom’s brother, then it would explain why he would want to hide Elsa’s powers. (His niece was kidnapped for her powers after all, by Mother Gothel!) Since both of these magical girls are blonde, it makes sense to say that magic has to do with a blonde trait. (Anna didn’t get any powers.)
Yes, Rapunzel was supposed to be a brunette as it turns out, and the only reason she’s blonde is because of the flower. Yes, all true. But! That doesn’t mean that the theory isn’t viable. Rapunzel’s genetics could have just been more susceptible to the magic of that flower. For all we know, the flower could have just awakened a power that Rapunzel might have had anyway if she had been born blonde. But because of the brown hair, the gene lay dormant. Anna, Elsa and Rapunzel look so much alike. Rapunzel and Eugene show up at Elsa’s coronation. We already have found a connection between the two films anyway.
And it makes sense that Elsa’s powers run in the family because her father seemed to know a lot more than he let on (knowing exactly what book to look into, knowing exactly how to get to the trolls, knowing the Troll King and vise versa.) I honestly think he may have had powers too. But to a weaker extent. He has dirty blonde hair, while Elsa’s has bleach blonde (and Rapunzel has honey blonde). I think her hair signifies how strong her powers are compared to her fathers. He may have powers himself, but much weaker than Elsa’s and that’s why he’s so dumbfounded on how to help her. At least, he doesn’t know how to help her completely because he doesn’t know how to deal with powers that strong. But, this is all just a theory and I can never completely prove anything. But, I don’t know..