He wants to marry the daughter of his sister’s husband
Question

                Can I marry the daughter of my brother-in-law (my sister's husband) but not my sister's child ?
Praise be to Allah.It is permitted, because she is not one of the mahrams mentioned in the aayah 
          (interpretation of the meaning): 
“Forbidden 
          to you (for marriage) are: your mothers, your daughters, your sisters, 
          your father’s sisters, your mother’s sisters, your brother’s daughters, 
          your sister’s daughters, your foster mothers who gave 
          you suck, your foster milk suckling sisters…” 
[al-Nisaa’ 
          4:23] 
What is 
          listed among these mahrams is “your 
          sister’s daughters”, and the girl – in this case – is not the daughter 
          of your sister. So there is no reason for this marriage to be haraam, 
          either on the grounds of blood, breastfeeding (radaa’ah) or being related 
          by marriage. And Allaah knows best.