The best advice I can give you is to leave things alone. You say you caught them in the act, but to they know that you know what they are up to? If they are blissfully unaware that you know they are having sex, then that is probably the way you should leave things.
I am sure that you are distressed because your brother is cheating on his dad, and dad's new wife is cheating on him with your own brother, but I want to counsel against you being the agent of their exposure. Though it is extremely unfair for people to react the way they will, I can tell you now that if you bring this to everyone's attention, the perversity of human nature will mean that you will end up being hated by everybody. Your stepmother will feel guilty for what she's doing, and will resent you for forcing her to confront her own guilt. Your brother will hate you, because he expects his sister to be on his side. Again he will resent you for spoiling his fun, and after his father has given him a thorough beating, or perhaps threatened to throw him out of the house, or whatever retribution may be due, he will hate you for causing this to happen to him. Your father, after the shock of discovering that his wife is cuckolding him with his own son, or feel resentment towards you for breaking up the family. Make no mistake, revelations like this invariably destroy the family totally.
The likelihood is that they will get caught, and it will result in a massive upheaval. But for your own sake, you don't want to be the agent of that up evil. If your dad is going to find out what's happening, you would be far better off if you could plead complete ignorance, and your reaction should be one of horror and shock. At least that way, you will still have your father's love, as he will not see you as being the one who was catalyst in the whole mess.
It isn't fair, but you would be caught right in the middle of it all, and that is the worst position to be in.
If they know that you know, the chances are they will stop what they are doing, or at the very least, they will behave with far more discretion. As it is, your stepmother is breaking the law, as the Indian penal code does not allow people to have sex below the age of 18. I'm not quite sure what the actual charge would be, but certainly having sex with a minor would see her in very hot water. But you don't want to be going to the police about this matter.
If you are determined to try to stop these things taking place, wait for an opportunity to speak to your stepmother alone. Tell her that you have seen what she is doing with your brother, and that she must stop it, as she is running the risk of destroying the entire family. Don't try threatening to tell your dad, as she may well call your bluff, and then you are left with no option but to put up or shut up. All you can really do is appeal to her sense of honour, so it would appear she doesn't have one.
I'm sorry if my advice doesn't really give you away of resolving this problem, but sometimes, there is no easy way of dealing with things. Better to leave them to make their own mistakes, rather than put yourself right in the middle of the whole thing.