What is the ruling on a deceased brother's paternal uncle, who is not a mahram to his nieces, entering their bedrooms and being alone with one of them, and what are the Sharia proofs that prohibit this?

The paternal uncle is a mahram to a woman, and it is permissible for him to travel with her, be alone with her (in khalwa), and enter upon her, provided there is no fear of fitna (temptation/strife). If there is suspicion or illicit transgressions on his part, then it is not permissible for him to be alone with his niece. No one is permitted to enter homes except with the permission of their occupants. As for the brother's wife, she is not a mahram to the paternal uncle, and she must observe hijab in front of him. It is not permissible for him to be alone with her or to see her unless she is in full Islamic dress. The paternal uncle is advised to fear Allah and avoid situations of doubt, and the sister is advised to obey her mother regarding not being alone with her paternal uncle, for obeying one's mother in what is good is obligatory.