Can anyone tell me where Cisanthe grandiflora sits, taxonomically
speaking. I've found mention of it in various horticultural contexts,
but no indication to which branch of the angiosperms it belongs.

From observation, it's a eudicot, with 5, free, petals, 5, free, sepals,
and a superior ovary. Presumably not an asterid, so either a rosid or a
basal eudicot.
--
Stewart Robert Hinsley