What is the ruling concerning the newborn and the marriage if the wife gives birth to a full-term baby five months into her marriage to her virgin husband?

If the five-month period mentioned is counted from the time of the marriage contract, and the child was born after five months, then the child is not considered your brother's son unless he claims him. The child is then attributed only to his mother, because the minimum gestation period is six months. The fact that she was a virgin does not change this ruling. However, if your brother claims him, the lineage is still not established according to the majority of jurists because it is considered zina (fornication), except according to Abu Hanifa.

If the five-month period is counted from the time of consummation or khalwa (private seclusion) with the possibility of intercourse, and the marriage contract took place at least one month before that, then the child is attributed to your brother, even if there is suspicion of it being from an illicit relationship, based on the hadith: "The child is for the firash (bed/marriage), and for the fornicator is the stone." Lineage is not denied except through li'an (oath of mutual imprecation).

It is not permissible to accuse a woman of zina unless she admits to it.

Regarding the ruling on marriage before or after childbirth: In the first case (marriage before childbirth), it is valid and there is no issue. In the second case (if the pregnancy is a result of zina): The Malikis and Hanbalis hold that it is not permissible to marry a pregnant woman who conceived through zina before she gives birth, whether from the fornicator or someone else, and the contract is void. However, the Shafi'is and Hanafis permit marrying a pregnant woman who conceived through zina, but if someone other than the fornicator marries her, it is not permissible for him to have intercourse with her until she gives birth.