Is it permissible for these girls to unveil in front of me and for me to speak with them, or are they not considered my sisters?

The daughters of your father's wife from another husband are not your mahram, and it is permissible for you to marry them. Therefore, it is not permissible for them to uncover themselves in front of you. Their obligation is to veil themselves from you and to be treated as non-mahram women.