What is the Islamic ruling on this?

okay so my friends dad, who is muslim got a Christian woman pregnant out of wedlock...then his relatives came and took the child away from the Christian woman and got the father married to another woman (who is Muslim) they had two kids together..today my friend is 17 and his brothers do not know that hes only their half brother...he goes to visit his real mom sometimes but he lives with his stepmom who is really evil...what is the Islamic ruling on this? Is the child at fault or are the parents...btw hes muslim too...so now what is the ruling on this?