No wonder you are confused. I wrote Karen instead of Maren. Ragnhild Gulbrandsdatter was daughter of Gulbrand Olsen and Maren Terjesdatter Gjennestad-eie. And in the 1801 census Nevesdal I guess someone has made a mistake. The wife of Jens Oudensen (43) should be Karen Terjesdatter (40).
Jens Ouensen was son of Ouen Jacobsen and Guri Gulbrandsdatter, who married 2 June 1753 in Øyestad (Maria besøgelsesdag) Øyestad, Ministerialbok nr. A 5 (1751-1757), Kronologisk liste 1757, side 115. Permanent sidelenke: http://www.arkivverket.no/URN:kb_read?idx_kildeid=9892&idx_id=9892&uid=ny&idx_side=-113http://www.arkivverket.no/URN:kb_read?idx_kildeid=9892&idx_id=9892&uid=ny&idx_side=-113 Ouen Jakobsen had a brother Salve, and Guri was sister of Gieruld and Gunhild. Ouen Jacobsen must have died, because Guri remarried 1765 to Even Halvorsen.
I have not found birth of Karen Terjesdatter. Probably Maren and Karen were not sisters. Marriage of first cousins was not common at that time. 50 years later it was very common.