Am I obligated to care for my deceased brother's wife and daughter?

The wife of the deceased brother has her share of her husband's inheritance, and she has the right to reside in his house until her waiting period ('iddah) ends. You are not responsible for her, rather her guardian is. As for your brother's daughter, after her father's death, her guardianship falls to her paternal grandfather. If he is not present, then to her father's executor, and failing that, to the judge. This guardianship is over her wealth. As for the guardianship over her person (her personal affairs), the executor takes precedence, then the grandfather, then the brother. If none of them are present, then it falls to the paternal uncle. Accordingly, you do not have guardianship over your niece's wealth, and the guardianship over her person falls to the paternal uncle in the absence of an executor, grandfather, and brother.