My mom had a son with her first husband which made him my step brother. My mother's brother (my auncle) had 2 daughters.... and they are my cousins as well as my step brother's cousin.Well Zubeida I had to enter all your family members into my FTM program to get the answer to this. First though let me correct you.
My father had 2 daughters with the 2 cousins (one by each of them), which made me also their sister. The one had a son and he wants to get married to my step brother's grandchild.
Are they related? If so, how? And are they allowed to get married?
You state that your mother had a son by another husband (not your father). That makes the son your HALF-brother, not your step-brother as you stated.
A step relationship is one that has no blood ties. But you and this son share your mother as a common parent and thus you are related by blood.
Okay moving on - your HALF-brother's grandchild is the third cousin to the grandson of the daughter your father sired with your uncle's daughter. So the two people wishing to marry are third cousins.
Whether or not it is legal for them to marry depends on where they live. You would have to check into the law in that country.