Is it considered severing ties of kinship if a brother's wife lives separately in her own home and refrains from visiting anyone, and I wish to see my brother and his daughters without seeing her?

Not seeing the brother's wife is not considered a severance of kinship ties, because the brother's wife is not among the kin whose ties must be maintained. Nevertheless, it is not permissible for Muslims to abandon and turn away from each other, so care should be taken not to boycott the brother's wife. If she insists on the severance, then she is the one who is sinful. The wife has the right to an independent dwelling, and she may refuse to live in a mixed-gender residence.