Alexandra/Aleksandra as a given name is out of the question. I found 5 in Vest-Agder county in 1865, upper-class women in Kristiansand town, none Serena combination.
The normal guess for Serena name is a rather common name: Serine/Sørine/Severine/Søverine (variations of the same name). But, - The spelling Seren is a strong indication that the names are kept in an oral tradition. So perhaps Siri is influenced by the name Seren and changed to Serena.

Siri Andersdatter had her last birth 5. Aug 1882: http://www.arkivverket.no/URN:kb_read?idx_kildeid=6190&i.... Anders died short after birth.
Siri died 28. July 1885: http://www.arkivverket.no/URN:kb_read?idx_kildeid=6190&i...

Ole Sørensen as a widower in 1910 census: http://da.digitalarkivet.no/ft/person/pf01036590000651
He was a retired tailor. Ole Olsen, born 1869, was his son-in-law.

I had hoped the Finsland bygdebok (farm history book) would give answer to the main question, but Søren Olsen Røyrås, born 1867, was not mentioned at all.