All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If the deceased person did not leave behind any relatives except those mentioned in the question, then inheritance will be confined to the father and the mother only, where his mother will get one-sixth of the inheritance because of the existence of a number of brothers and sisters. Allaah The Almighty Says (what means): {…and if he had brothers [and/or sisters], for his mother is a sixth…} [Quran 4:11]
The remainder of the inheritance will be for the father by Ta’seeb (by virtue of having a paternal relation with the deceased and not having an allotted share, so he gets what is left after the allotted shares have been distributed), for the Prophet, sallallaahu ‘alayhi wa sallam, said: “Give the prescribed share of inheritance to those who are eligible for it and the remainder [of it] is for the nearest male blood relative.” [Al-Bukhari and Muslim]
As for the other relatives such as brothers, sisters and nephews, they will get nothing because they are prevented from inheriting by the existence of the father of the deceased.
Also, the missing step-mother is entitled to nothing of the inheritance, even if she is present, and thus being missing has no effect on dividing the estate.
Allaah Knows best.