its normal, happens all the time. go look up " GSA" ( Genetic Sibling Attraction).. as for "god" letting this happen.. he didnt let it happen bc he does not exist..
I also had similar incest feelings when I was in my teens. But my sexual object is not my sister, but my mother. I used to fantasize about her a lot.
After many years(now I am 27) i realized the feeling of guilt in me and had the same guilt feelings which you are facing now. I blame god for what he let happened and why he chose me. But as I reasearched more on the net, I came to understand that in teenage, especially in males, sexual hormones are raging and they are automatically attracted to the closest possible opposite sex partner in my case it happened to be my mom. Now I dont fantasize her anymore, and I do love, care 7 respect her very much like every other son. So, my understanding is its normal in teenage to have aroused sexual feelings which cant be controlled.
So rather than getting depressed over what you have done, try to understand and analyse why & in what situations those thoughts occured to you. Its wrong if you continue the same thoughts even after you know that its wrong. Fantasizing is our mind thing, but it should not hurt the sentiments & beliefs of others by acting on it.
All the best.
The sex drive of 15-17 year olds is very strong and males that age are looking (sometimes desperately) for any sex object to release the tension. This is driven by powerful hormones that begin to flood into your bloodstream and can be almost uncontrollable. In your case, with limited or no contact with other members of the opposite sex, it is not at all unusual for you to consider female siblings as possible sexual partners. But YOU MAINTAINED CONTROL and acted on your impules only in a very limited way. Rather than feeling guilty you should proud of that.
I think you should go talk to some one about your feeling. God wouldn't want you to end your life over anything.
And you should pray to god to help you threw this .