Canon Law states:
Can. 1059 “Even if only one party is Catholic, the marriage of Catholics is governed not only by divine law but also by canon law, without prejudice to the competence of civil authority concerning the merely civil effects of the same marriage.”
Only Catholics who have left the Catholic Church by a formal act are excepted by Canon 1124:
“Without express permission of the competent authority, a marriage is prohibited between two baptized persons of whom one is baptized in the Catholic Church or received into it after baptism and has not defected from it by a formal act and the other of whom is enrolled in a Church or ecclesial community not in full communion with the Catholic Church.”
Can. 1066 “Before a marriage is celebrated, it must be evident that nothing stands in the way of its valid and licit celebration.”
His fiancee’s previous marriage is presumed valid until determined otherwise. He cannot marry someone who is presumed to be already married.
Can. 1108 §1. Only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses according to the rules expressed in the following canons and without prejudice to the exceptions mentioned in cann. 144, 1112, §1, 1116, and 1127, §§1-2.
The issue concerning whether you should attend your brother’s wedding is complicated. Please read the Catholics United for the Faith article Should I attend?