If you search Wikipedia or Erowid for the chemical structures of 2Cs and NBOMes, you'll find that while the NBOMes contain the core 2C skeleton, they have an additional aromatic ring on the amine part of the phenethylamine core. That is enough to significantly change their absorption profile, tendency to cross the blood brain barrier, and affinity for the target binding site. So while they are related, it's not "siblings" like the 2Cs are to each other. It's more like "cousins".