Charan
Don't worry and just chill and read below points.
1) Did you see the evidence of adultery as reported by her your self? If not then she might be bluffing or might be having some proof like your brother has stayed with some one in a hotel but she might not have proof like your brother has done intercourse and If I am not wrong then Adultery requires a willful act of intercourse. So first find out what proof she is having...otherwise it would just be considered extra marital relationship at the most.
2) Even if she has proof suggesting that your brother has done intercourse/or had relations, she won't be able to convict him under 498a or DV and can't imprison him...but she would be able to justify her stand of staying away from your brother in CRPC 125 and other maintenance cases and your brother would have to pay maintenance to her based on his income and expenses. Moreover she would be able to get divorce on this ground.
3) If they have proof of 25 lacs given to you then only return those money otherwise pressurize them to settle the matter or say you won’t give this money back until court orders and deny court of any such transaction. IN ANY CASE DON’T confess in court about such transaction unless you know that they have proof. NO QUESTION OF GIVING 1.5 CRORES…SIMPLY SAY THAT YOU WOULD GIVE AS PER COURT’S ORDER.
4) Be in touch of this forum and you would know many things (related to your case as well as otherwise).
5) Finally there is no law who can book any one for adultery (UNLESS THE GIRL WITH WHOM YOUR BROTHER WERE CAUGHT IS MARRIED AND HER HUSBAND COMPLAINTS).