Where does “kaksossiskokset” come from?

kaksossiskokset (Finnish) comes from Finnish siskokset, from Finnish Sisko, from Finnish sisar, from Proto-Finnic sesar, from Proto-Balto-Slavic swésō, from Proto-Indo-European swésōr, from Proto-Indo-European su-h₁ésh₂-ōr, from Proto-Indo-European h₁ésh₂r̥ — blood.

Ancestry of “kaksossiskokset”, step by step

kaksossiskokset traces back through two separate lines of ancestry.

via Finnish siskokset

StepLanguageWordMeaning
1Finnishsiskoksetsisters collective term for people who are each other's sisters; chiefly used of family or family relations
2FinnishSiskosister
3Finnishsisarsister
4Proto-Finnicsesarsister
5Proto-Balto-Slavicswésōsister
6Proto-Indo-Europeanswésōrsister
7Proto-Indo-Europeansu-h₁ésh₂-ōr
8Proto-Indo-Europeanh₁ésh₂r̥blood

via Finnish kaksonen

StepLanguageWordMeaning
1FinnishkaksonenA twin; When used as modifier in compound terms...
2Finnishkaksitwo
3Proto-Finnickakcitwo
4Proto-Uralic*kakta
Every word from Proto-Indo-European h₁ésh₂r̥