In the 19th century there were certain geographical barriers to travel in Iceland ... crossing some of the rivers was a bit of a challenge. In fact, some of the barriers lasted longer than others - the last stretch of highway 1 - the ring road around the country was not completed until 1972.
Anyhow. Margrét Sigríður Jóhannsdóttir was indeed the first wife of Hjörtur. She died in 1882, and Hjörtur re-married (to Ólína Ágústa). As the custom was in Iceland, one of their daughters was named after his first wife, so you had Margrét Sigríður Hjartardóttir, born in 1890. She actually stayed with her mother when the family was broken up.
As as side note, there are 11 grandchildren of Hjörtur still alive here in Iceland. They are in their 70 or 80s, and I have no idea if they know anything about what became of their aunt Sigrún, who emigrated, but it might be worth a try to contact them.