I am unable to narrow it down to Sankhya or yoga or something else?
Brahkumari site does not name any individual as Guru of their fouder.So they do not seem to have a Guru-parampara.Normally the school is judged by the lineage. It seemes that it stated with their founder. So far as I know, they have revised the Gita also. So I do not think they belong strictly to any classical or Sanatana school.