lmp and
Celuien, I love the idea!
Celuien, Sæthryd is older than Leafa by a few years, in which case, even if they lived close by one another, Leafa wouldn't remember the incident of Sæthryd's disappearance. My thought is that perhaps Leafa's father was partly inspired to become a wayfarer because he was ashamed of Sæthryd's wild wanderings. I picture him as the fellow that would be deeply effected by something like that. He's a very proud man, in a very odd way.
I imagine they would have left while Sæthryd was still quite young and Leafa was little more than a baby, and, that is to say, they left the immediate vicinity. If my memory of Leafa's history serves me correctly, they didn't go off with the wayfarers until much later. As a child, Leafa would have no doubt heard her father tell the story many times, and it would have made a deep impression on her.
Briefly put, the two families lived close to one another up until the time when Sæthryd went mad, at which point Leafa's family left, as her father became ashamed of the connection. Leafa would have no memories of Sæthryd, but would have heard much about her. Does this sound all right with you? As for the precise nature of their connection, I have no strong feelings about it. Would cousins do?
lmp, thanks for the wonderful suggestion. I'm beginning to feel inspired again.