What is the ruling on DNA testing to prove or disprove the paternity of the two girls to my brother, and is this test considered legitimate evidence in this case, given that the killer claimed them as his own, and doubts surround their lineage after my brother's death?

The child belongs to the bed, and no one has the right to deny a person's lineage to his father except the father himself through mula'anah (oath of condemnation). If the father dies, it is not permissible to deny the child's lineage to him. Whoever denies someone's lineage to his father without proof is a slanderer. There is no need for a DNA test, for the two girls are, Islamically, your brother's daughters, even if the mother and the fornicator admit to fornication. This is based on the Prophet's (peace be upon him) saying: "The child belongs to the bed, and for the fornicator is the stone."