First, Finnur had one maternal half-brother. Jóhann, Finnur's father died in 1863, but but in 1866 his mother had a son, Jóhann Jónasson. He married Guðbjörg Valmundardóttir and emigrated in 1887, but I have no idea of what became of them.

Finnur had also 5 half-siblings on his fathers side, His father's first wife was Guðríður Ívarsdóttir (1811-1852). Their children were: Karitas (1832-1834), Þuríður (b.1841, disappears from records - probably died young), Jón (1842-1897, farmer in Stóri-Langidalur, has descendants in Iceland), Krostín (b. 1843, disappears from records - probably died young) and Kristján (1844-1884, has descendants in Iceland)

As for Herdís, her siblings were:

Guðrún Msrgrét, born 1859, married to Jón Daníelsson, one child.

Guðmundur, 1863-1905, married to Halldóra Jónsdóttir, 4 children.

Eyjólfur, 1866-1958, married to Sesselja Pétursdóttir, two children.

Elínveig, born 1868, married to Jón Sigurðsson, no children.

Kristjana, 1875-1933, married to Jón Einarsson, one child.

All the information above is taken from the online 'Islendingabok' database.