If he marries a woman and consummates the marriage with her, he becomes a mahram to her daughters and granddaughters
Question

Someone’s father was married to two women, and his father had daughters from his second wife. His father died, and another man came and married his father’s wife who was that person’s maternal aunt. Are his sisters through his father mahrams to the husband of his maternal aunt or not?
Praise be to Allah.If a man marries a woman and she has daughters, then he becomes a mahram to them when he consummates the marriage with their mother, because Allah, may He be exalted, says, explaining who are a man’s mahrams (interpretation of the meaning):
{…and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you} [an-Nisa’ 4:23].
The step-daughter is the daughter of one’s wife. It is not stipulated [in order for her to become a mahram] that she be under the guardianship of the new husband, because the verse refers to what is usually the case, so it is not stipulated.
The fuqaha’ are unanimously agreed that the step-daughter becomes a mahram to her mother’s husband if he has consummated the marriage with the mother, even if the step-daughter is not under his guardianship. Tafseer al-Qurtubi (5/101).
It says in Fataawa al-Lajnah ad-Daa’imah (17/367): If a man marries a woman and consummates the marriage with her, it becomes permanently prohibited for him to marry any of her daughters or granddaughters, no matter how far the line of descent reaches, whether they are daughters from a previous or subsequent husband, because Allah, may He be glorified and exalted, says (interpretation of the meaning):
{Prohibited to you [for marriage] are your mothers … and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you} [an-Nisa’ 4:23].
What is meant by the step-daughter is the daughter of the wife. So the man is regarded as a mahram of the daughters of the woman whom he has married and consummated the marriage with her, and it is permissible for them not to observe hijab in front of him. End quote.
Based on that, in the case of the man asked about, if he marries the woman and consummates the marriage with her, he becomes a mahram to all of her daughters and granddaughters, if there are any.
And Allah knows best.