There are very few sources that mention Sif as a jotunn. Most are adamant about Sif being an Aesir goddess. Perhaps Sif’s connection with the earth must have misled some of the old authors. Moreover, Sif is referred to as a rival of Jarnsaxa, a jotunn with whom Thor had Magni. This could have worked as a support for the theory of Sif as a jotunn. Yet, the vast majority of the old texts agree that Sif is a goddess and not a giantess (jotunn).