Praise be to Allah.
Having children is something that is decreed by Allah, may He be exalted, as He says (interpretation of the meaning):
“To Allah belongs the dominion of the heavens and the earth; He creates what he wills. He gives to whom He wills female [children], and He gives to whom He wills males
Or He makes them [both] males and females, and He renders whom He wills barren. Indeed, He is Knowing and Competent”
Among a single family there may be those who can have children and those who cannot, and the reason for that may be with the man or the woman.
You mentioned the paternal aunts, but you did not mention the sisters, maternal aunts, or paternal cousins.
So long as the girl is good in terms of religious commitment and character, and [your brother] is pleased with her, you should not mention this, because it might prompt him to cancel the marriage, or to proceed, but with anxiety about the future.
But if most of her female relatives could not have children, or that is something that is clearly well-known among them, then you must tell your brother in that case, so that he will be fully aware of the matter.
Islamic teachings encourage men to marry women who are fertile, meaning those who are able to bear many children.
It was narrated that Ma‘qil ibn Yasaar said: A man came to the Messenger of Allah (blessings and peace of Allah be upon him) and said: O Messenger of Allah, I have found a woman who is of good lineage and beautiful, but she cannot bear children; should I marry her? And he told him not to do that. Then he came to him a second time, and said something similar, and he told him not to [marry that woman]. Then he came to him a third time and said something similar, and the Prophet (blessings and peace of Allah be upon him) said: “Marry women who are loving and fertile, for I will be proud of your great numbers [on the Day of Resurrection].”
Narrated by Abu Dawood (2050) and an-Nasaa’i (3227); classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him) in Adab az-Zafaaf (p. 132).
The fuqaha’ stated that it may be known whether a woman is fertile by looking at her relatives.
It says in Kashshaaf al-Qinaa‘ (5/9): It may be known whether a virgin is fertile by seeing whether she comes from a family whose women who are known for having a lot of children. End quote.
See also the answer to question no. 32668.
And Allah knows best.