To the OP: I am going to make a few things very clear to you, and I hope you do well to take into consideration what I am about to say.

I am going to be very clear to you that even if this was caused by your medication for some reason, you are still responsible for your actions and the consequences that will ensue. This is like saying you had sex with a girl who, for some reason or another, was in no position to give consent, but it was okay because you were drunk and you don't really remember doing it anyway. It still happened, and it is still your responsibility, because you still chose to drink.

What I am saying is that this could have been prevented, and it can be in the future. I know this because I know how medications work. You know your body. You know when something is "off," and you can often tell if it is medication related. Even if you don't know, the following still applies, and you should still make the call to a professional:

When ANY medication is starting to do something unusual, it's time to call your doctor. That doesn't mean in a day, or a few days, or a few weeks. That means NOW. Whether it is a psychiatrist or a different kind of doctor, they will give you their emergency number when you have your first appointment with them. They have to. If you call them in an emergency, the law requires that they respond to your call in no more than four hours. And they tell you, no matter who the doctor, that if this is an emergency and they cannot be reached, you go to the emergency room. That includes if you feel you are a danger to yourself or others. As a minor, both you and one or both of your parents should have been informed of this at the time you were first seen by your doctor.

It is important to realize your medications did not make you do this. There is no known medication that can make someone commit an act of sexual abuse. Can they impair your reasoning and other cognitive functions? Sure. But as stated above, the second you notice that you call the doctor, and it's pretty hard not to notice a side effect as serious as "blacking out" or other cognitive distress.

There are medications that can increase sexual libido, but it is highly unusual for these to be given to fourteen year old boys for obvious reasons. And if they are, your doctor would be monitoring you much more closely. It is highly unlikely that these were medications you had been stable on for a long time, so your doctor should have been watching you closely anyway. Medications often go through more changes in adolescence than at any time so as to compensate for changing hormones and mental growth.

Also, as a side note, any father who beats his children and any mother who thinks her daughter being sexually abused is no big deal are unfit for parenthood, and you and you sister should be removed from their custody.

In essence, you may have had medications that were faulty, but you cannot blame this on them, nor on being in a blacked out phase. You still made a choice, and you made a choice to do something that was immoral and wrong and will most likely have life long consequences for both you and your sister.

Does this mean you should go into the depths of despair and think you are a worthless, pathetic human being? No. But this is not a situation where you can comfort yourself with the fact that what you did was only a little mistake and can be soothed away. You screwed up, pretty badly. And you need to own up to that. I personally would like to see some legal consequences ensue from this to drive the point home, but it seems that may not happen, and perhaps you can be in charge of your own discipline.

First of all, you apologize the shit out of apologies to your sister. You make amends in every way possible. You get down on your proverbial knees and do it. You apologize harder than you ever have in your life, and hopefully the hardest you will ever have to. You don't just do it with words, either. You find someway to show it with actions, having it preferably include going to specialized counseling for this. Can't afford it? There are plenty of people who will help you out with that. Talk to your school guidance counselor, anonymously if you want, about ways to find financing or a program that will help you attend such a type of therapy.

Even if you don't go to therapy, you get to the bottom of what happened, and don't leave it with "crazy reasons I can't explain." There is a reason in a compromised state you chose to molest your sister. That's not exactly a "go-to" thing when people black out. And you need to make sure whatever happened never happens again. Because maybe this time it could be passed off as an innocent mistake, but if it happens again you won't be so lucky.