It's a mix of facts and fiction. I ship Jelsa, but not for the same reasons as many. I like it because it is actually tragic. Jack Frost and Elsa would never be a couple or meet and here's what I think is why.
1695 - Jackson Overland is born in a village soon to be called Burgess, PA.
1702 - Jackson's sister is born.
(Okay I consider Jack to be 17 and his sister 10 if you want to go off what they said in the movie or other sites then fine 1698 Jack would be 14 at time of death and 1704 sister would be 8)
1712 - Jackson Overland drowns in a pond. - Jack Frost is born.