1)Allah directs (them) about those who leave no descendants or ascendants as heirs. If it is a man that dies, leaving a sister but no child, she shall have half the inheritance. If (such a deceased was) a woman who left no child, Her brother takes her inheritance. If there are two sisters, they shall have two thirds of the inheritance (between them). If there are brothers and sisters, (they share), the male having twice the share of the female. Thus doth Allah (swt) makes clear to you (His knowledge of all things). [An-Nisa 4:176]
2) in your case If there are BROTHERS and SISTERS, the male will have TWICE the share of the female as rightly directed by court
3)Nephews and Nieces are ONLY entitled in the absence of Brothers and Sisters. This means that they take the shares of the Brothers/Sisters of the deceased in their absence. Hence a Nephew/Niece will receive what his/her parent (Brother/Siuster of the deceased) would have received if he/she was alive