Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Yes, it is true that a Muslim does not inherit a Kaffir and vice-versa. Bukhari, Muslim and others reported that Usama Bin Zayd (Radiya Allahu Anhu) narrated that the Prophet (Sallallahu Alaihi wa Sallam) said: “A Muslim does not inherit a Kafir and a Kafir does not inherit a Muslim”.
However, it is permissible for a Kafir to offer a present to a Muslim and vice-versa.
But it is not permissible to inherit someone except after his death. Nevertheless, if your stepfather is not declared legally incompetent because of an illness or the like, then there is no objection that you accept what he offers you, but this should be as a present not as part of the inheritance.
Anyway, you are not a heir to the wealth of your stepfather even if he is a Muslim. A son or daughter does not inherit a stepfather because he is not his/her real father.
Allah Says (interpretation of meaning): {…nor has He made your adopted sons your real sons. That is but your saying with your mouths. But Allâh says the truth, and He guides to the (Right) Way.}[33:4]. You cannot be related to anybody except to your father.
Allah knows best.