According to the publication paper by S. Rahayu and M. Rodda, Hoya sulawesiana is most closely related to Hoya isabechanae. They are also both in the Acanthostemma section of Hoya and are both endemic to Sulawesi Island. There are however many differences with the biggest being the leaves of the two species. Below the flowers of both species. H. isabelchanae on top and H. sulawesiana on the bottom: