بسم الله الرحمن الرحيم
(Fatwa: 1212/1211/M=10/1434)
If the girl whom the childless couple has adopted is not their Shariah heir (waris) then it is lawful for them to make a will in one third of their wealth and it shall be enacted. What do you mean by settling the deed? If the real sister of deceased couple has claimed for her that she is legal heir then this claim may be valid though they did not have good relations their whole life. In order to know who are the real heirs of the deceased, you should first write the details of deceased close relatives that who among the parents, brother, sister, uncle, nephew etc are alive and what is their number, then the real heir may be pointed out.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband