First, stop and take a deep breath, okay? You aren't a waste of space, and you aren't some big, horrible monster. Reading over your first post carefully, and listening to some of the replies you've made, I think it's more likely that, like someone else already mentioned, this is more of a case of teenaged hormones and misplaced attention.
It's natural for boys in your age bracket to be aroused by just about anything, from the least provocation - that doesn't mean acting on it is right. Yes, you've made a few mistakes - but its nothing that cannot be worked on and dealt with. You do need to find a way to come clean about this, or to discuss it with your parents or someone else who can help you cope with the feelings your having and the things your going through.
I'm sorry that you feel like you are unable to talk openly with your parents about this issue - but can understand that you may not have a safe environment to discuss it in. Is there any other way you could get into youth counseling or therapy? Perhaps raise the issue of depression as a side note with your doctor for a referral, and find help on these other issues that way? Normally I would not suggest lying, but if you have no other alternatives to getting help, sometimes it's necessary.
The issues with your sister do need to be resolved, though, and honesty is always the best policy, if you are able to find a way to do it.
Know, however, that you are not a horrible person, you are not a monster, and you are not a waste of space. You are a valued, living person who deserves a happy life - no matter what kinds of mistakes they've made.
I hope you are able to resolve this issue!
-PSORLY