I have come upon some notes I wrote with information that my mother gave to me concerning my father's grandmother Margaret Hartin.
She said that she had an affair with a man and left her young family to go and live with him. No names were mentioned.
I am positive that this information will be correct. I hope that this information will help to sort out this issue for you. Have you contacted your brother re the marriage and birth certificates for Margaret Hartin.
I have found information on the forum that Fell's came to Australia. Unfortunately we have just moved from the area where some of them settled in NSW.