I have no child even after the 13 year of my marriage. I live in pakistan. My sister is living in england and have british nationality.her husband is the real brother of my husband.they have three daughters and they are agree to give us one daughter of two year. Is there any law can we adopt my niece leagally so that she live with us in pakistan

Sorry but I cannot see a legal way in your case.
1. Adoption is forbidden in Pakistan/ if I am not mistaken/
2. You cannot come to private agreement in the Uk in this case
3. Adoption of subjects of Her Majesty from foreigners is not supported by the British government and law.