According to Fortson's Indo-European Languages and Culture, the Proto-Indo-European form for "grandson, nephew", *nepōt- became the name for the Indo-Iranian deity whose name means "grandson of the waters" - Vedic Apám Nápāt, Avestan Apąm Napā̊. He also connects it to the Irish deity Nechtan and the Roman god Neptūnus "Neptune". (The OED doesn't accept this derivation of Neptune.)
*nepōt- became Latin nepōs "grandson", becoming Anglo-Norman neveu, borrowed into English as nephew. The Latin feminine form neptis became Anglo-Norman nece and English niece.
There is a theory that naphtha is derived from Apám Nápāt thru Greek. This is unlikely; naphtha is either from Semitic or perhaps from PIE *nebʰ- "cloud".