Your question itself is not correct. You had already assumed it is wrong, and questioning why it is so? You could have asked "Is incest wrong? If so why?"
You are not thinking or reasoning. You are only expressing your feeling towards it.
Please if possible try reasoning it
Incest is not only a taboo for humans. Many of the social animals in wildlife AVOID it. Male youth are chased away to find mates from other colonies. You can observe this in lions, hyenas, dogs, wolves, elephants etc. I had seen several videos mentioning this fact on discovery/NGC/AnimalPlanet.
But sex for animals is only to procreate. I dont think other than humans and dolphins, any other animal mates for pure pleasure.
Having said that if INCEST, INBREEDING and SEXUAL ABUSE are understood clearly as different, I dont think incest is wrong.
Now why is that everyone here relates incest to brother sister? Do you consider sexual activity, relationship between father/dauther, monther/son too? Because being very clear about what you treat as INCEST is very important to reason, express it properly.
Example some one may say brother-sister incest is ok but mother-son incest is sick!
I personally think any form of incest between close family members with mutual consent, understanding and sound relationship, which doesn't lead to inbreeding, and only serves sexual exploration, pleasure, fun and intimacy.
But fact is I never had slightest of sexual attraction towards my mom, my fictious sister. In fact I adore them for their nature, style, capabilities and kindness towards me. I get flattered the way they remember me and associate me to certain things in this world. But never been sexual. But the strangest thing is I read erotica that involves incest and find it hot.
Guess our mind is the most difficult thing to know completely and understand fully.
Sorry it is a long post