My 3rd Great Grandmother, Marija Grbic (nee Sorco-Machiella)

https://www.geni.com/people/Marija-Grbi%C4%87-Sorco-Machiela/6000000006954332032

was born 25 August 1830 in Vručica - Trpanj, Pelješac, Hrvatska (Croatia)

and died Živogošće, Splitsko-dalmatinska, Hrvatska (Croatia) on 17 February 1915.

I am trying to find her relationship to her sister-in-law, Mande Grbic (nee Sorco-Machiela

https://www.geni.com/people/Mande-Grbi%C4%87/6000000006890492385

who was born 1822 in the same place as Marija. She died 28 March 1888 and was married to Marija's husband's brother. I have links to Mande's brothers who used the Macela surname and their father, Franjo Sorco-Machiela (Macela).

https://www.geni.com/people/Franjo-Sorco-Machiela/6000000006959718175?through=6000000006890492385

I strongly suspect that they may be sisters and presume a Venetian family lineage may be possible. 

Can anyone suggest some avenues of research?

Thanks, Michele