Question Summary:
Can the stepchildren marry each other?

Question Detail:

Asalamualaikum My question is quite complicated so please read it carefully . One of my friend (ABC) father(FFF) died long back and so his mother(MMM) married another man(xyz) . Now this man(xyz) was also married before and his wife(RRR) died .(In short both MMM and XYZ are widow) This man(xyz) had one daughter(PQR) with his first wife(RRR) . Now my friend(ABC) is going to marry this girl (PQR) . Is this marrage valid or haram . Jazakllah

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
 
As-salāmu ‘alaykumwa-rahmatullāhiwa-barakātuh.
 
It is permissible for ABC to marry PQR.[1]
 
And Allah Ta’āla Knows Best
 
Saanwalibn Muhammad,
Student DarulIftaa
UK   
 
Checked and Concurred by,
Abdul Azīm bin AbdurRahman,
Ismail Desai.