People's bodies respond differently to sexual abuse and incest, but bodily response is irrelevant. The only things that are relevant is consent and legality.
If you did not consent to the sexual contact, then it was wrong. It was a crime. Some criminals will try to convince you that the orgasm proves that you consented, and some ignorant judges or juries may believe it. But it simply IS NOT TRUE. Our bodies are made to respond to touching and contact even under severe duress. This does not mean you are in any way responsible or that you participated willingly. Incest most often happens to small children, who are in no way capable of providing consent to a sexual act. An orgasm is not considered consent. It was an involuntary reaction by your body, like a sneeze or a hiccup.
To illustrate the principles of consent, I will use a slightly less horrible example. A woman was being relentlessly pressured by her husband for sex. He drove her crazy. She tried to talk to him about it, but he would not accept her viewpoint, and kept bothering her. So eventually she would relent just to get some peace of mind.
After the sex, he always pointed out that she had also had an orgasm. He took it as proof that she had wanted sex too, and proof that he was doing nothing wrong.
But he was wrong.
In this example, the wife relented, which means that it was not a criminal act. But in a healthy relationship, sex is an act of love that celebrates a relationship. Afterwards, you should feel good about yourself, your partner, and the fact that you are together. But to this woman, sex was a chore. Afterwards, she felt lousy about herself, lousy about her husband, and was seriously questioning their entire life together. Her orgasm had no relevance to the fact that this was a seriously unhealthy relationship.
In the context of incest, abuse or sexual assault, the situation is even worse. In these instances, incest and the sexual abuse isn't even about sex. Sex is merely the weapon used by the attacker to demean the victim as much as possible. In Old Testament times, soldiers of a victorious army would often rape the soldiers of the losing army -- not because they were gay, not because they wanted sex, but because rape was the ultimate humiliation, the ultimate act of domination and violence. And everyone knew it.
It's understandable that survivors of incest, abuse or sexual assault would be confused by their body's response if an orgasm occurred. But make no mistake about it -- that does not make it a sexual experience. It does not imply consent to sex or enjoyment, and does not negate the horrible emotional wounds that are inflicted by incest. It was an involuntary bodily reaction, like a cough or a sneeze. It should not be interpreted in any other way.
If you had an orgasm or response as a result of incest, please do not let that stop you from getting help in healing from the crime that was committed against you.