All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The girl that was adopted by your uncle has no share in the inheritance; adoption does not entitle a person to inheritance or to have the same family lineage (of the adopting family), and this was abolished by Islam as we clarified in Fataawa 82371 and 84422.
However, each heir has the right to ask for his/her share of the inheritance as this is a right that Allaah allocated to him/her. The fact that the wife of the dead is a good person and needs money does not allow her to prevent other heirs from taking their share of the inheritance. The solution is to give each heir his/her right unless he/she renounces his/her right with his/her own will while being a pubescent person.
Therefore, if your uncle had died and left behind a wife and two full-brothers, or two brothers from the father’s side only, and he did not leave any other heir, then his wife gets one-fourth of the inheritance and the rest will be for his brothers as there are no other heirs.
Allaah Knows best.