1. Your cousin's wife has committed fraud by concealing her age, but she cannot be prosecuted under s.420.
2. By getting the matrimonial alliance made by concealing her age this girl has committed fraud, which entitles her husband to seek the nullity of the matrimonial tie by filing a petition to this effect in the court.
3. However, more advisable recourse is to apply for mutual divorce by drawing up consent terms and filing it in the court. The issues of alimony, compensation, etc can be amicably settled among the spouses in the MOU. If, however, she does not agree to mutual divorce then your brother may unilaterally move to court.
4. Your brother should contest on merits the 498A case filed against him.
5. He can also sue his wife for damages as she lied about her age to him.