Is the daughter of my second grandfather's wife considered one of the Mahram, or is she permissible for me to marry?

The granddaughter of your grandfather from his second wife is considered a mahram. She is either an paternal aunt (your father's sister from his father's side) if she is the daughter of your paternal grandfather, or a maternal aunt (your mother's sister from her father's side) if she is the daughter of your maternal grandfather. Paternal aunts and maternal aunts—whether full sisters, half-sisters from the father's side, or half-sisters from the mother's side—are forbidden for marriage, based on the Almighty's saying: "Forbidden to you are your mothers, your daughters, your sisters, your paternal aunts, and your maternal aunts." Even paternal and maternal aunts through suckling are considered mahram.