The birth records for both children list Per Jonsson & Martha Ericsdotter as the parents: Kerstin's birth record is at bottom of page 43 at https://app.arkivdigital.se/volume/v30809?image=25 Jon's birth record is the 3rd one down on page 68 https://app.arkivdigital.se/volume/v30809?image=38
Erik Nilsson's granddaughter Kerstin is later shown in the 1784-1788 household records with her husband Pehr Ericsson https://app.arkivdigital.se/volume/v18250?image=51
So, I think Erik Nilsson had another daughter (Martha) who was born much earlier (1734). Maybe Erik Nilsson was married before. There were no birth records in Vigåsa that fit around 1734. But, there was a birth record in Vrå in 1732 for a Martha born to an Eric and Kiersten in Ljushult, Vrå (if I'm reading the place correctly):
Ljushult, Vrå was where Erik Nilsson and Segrid's first daughter, also named Kiersten, was born. So, it's possible. I still need to "prove" the connection to Erik Nilsson but think I'm on the right track. And, I have the other grandchild to track down.
An interesting thing, when you mentioned errors are possible in the records - when I was researching the marriage records for Martha Ericsson and Per Jonsson - the only one I found was the 1758 marriage record in Vigåsa for Soldier Per Nilsson at:
I'm wondering if this should have been Per Jonsson.
If you have time to take a look at what I've sent :) (I know it's a lot) I'd appreciate your comments, especially if I've erred in any of my assumptions.
Thanks to all who have helped with this - I appreciate it more than I can say.