Dr. Bilal said that Nusairis (extreme shi'a) are a branch of Nizaaris, but I have read that it’s a sect from the Seveners. Which one is correct?

Connections have been made between Nusairism and Isma‘ilism of which the Nizaaris and the Seveners (Sab‘iyyah/Waaqifiyyah) are sub-sects, as well as between Nusairism and Christian Gnosticism. However, there does not seem to be any conclusive evidence.

Please refer to the relevant pages of Mislamic Sects by Dr. Bilal Philips for his discussion.

It is not correct to consider Nusairis extreme Shi‘ah since their beliefs are far worse to the extent that even scholars of the Shi‘ah have excommunicated them in the past. Rather, because of their denial of basic Islamic teachings using esoteric interpretation, the Nusairis are placed under the Baatineeyah category along with the Isma‘ilis, Druze and others.

And Allah knows best.