Really bad dreams.

The post you have written has a lot to unpack.

You seem to feel that a thought you experienced upon waking MIGHT represent a repressed experience from an earlier point in your life -- perhaps a sexual experience of sorts with your older brother. Assuming you are right, and that it is not simply the memory of a waking dream unattached to any reality, then there are a couple of things to talk about. (The brain seems to use dreams to sort through and file experiences and thoughts from the days activities. These are notoriously difficult to connect to any reality, or to interpret correctly, as they only mean something to the internal circuitry of the brain itself.)

First, let's say that it actually IS the surfacing of a repressed memory of a childhood event. At this point, what is the tangible result (beyond your consternation), and what actions does it merit? If the event you describe happened as you believe, then it was likely the sexual explorations of your brother's developing sexual awareness. Perhaps he had heard of such things talked about by his peers, and was curious about reenacting what he believed he understood the experience to be. Despite your feelings about it, it is relatively normal for siblings to experiment on each other as they approach puberty and begin to develop curiosities and thoughts of a sexual nature. This is often how humans learn about their own sexuality. You react to the idea as if it were coerced or forced, which it MAY have been, but not necessarily. The disgust you voice about it is something you have been TAUGHT to feel, likely by other family members or church community, perhaps? The act of kissing your brother's backside, in and of itself, seem relatively without risk of harm or danger to you. And children often experiment with such behaviors with no resulting harm or condemnation. The degree of your self-loathing seems unnecessary and unwarranted, as no actual harm would have been done to your young self by such a thing. You are looking at it through the lens of your current age and knowledge, hence your feeling of embarrassment and disgust. Were you speaking of say, an act of forced sodomy for example, we would be speaking of it quite differently. So it is a question of degree. At a young age, little trauma would likely have resulted from this.

I am somewhat more concerned here that you are so quick to condemn yourself for even having the thought at all. You seem to feel that those around you could somehow sense that you have had this vision, and will look at you with revulsion or disgust. Even IF you secretly found the image and idea of such activities to be darkly appealing or sexually exciting -- even supposing you were able to acknowledge this in yourself -- those feelings and thoughts truly do not make you a bad, weird or sick person, they would only make you a sexual being like to rest of us. Your worth, both to yourself and to the world at large, must be based on the content of your heart and your treatment of others in the real world.

The other thing to consider, IF this is a memory of an actual event, is what would you do about it now? What COULD you do about it now? The only thing within your control is how you respond to this vision. You could simply ignore it and chalk it up to a weird, bad dream that you remembered upon waking. OR, you could acknowledge that it MIGHT have happened, and respond in a rational and reasoned manner. Most of all, you would do well to FORGIVE yourself for having a thought of which you might not approve, and -- if true -- forgive your brother for engaging your younger self in such relatively innocent explorations. Either way, there is little use in agonizing over something that may not even have happened. Be a better parent to yourself, and absolve yourself of responsibility for what your mind may create without any prompting from you. Value who you are and who you want to be, and go forward unencumbered by any doubts about your fine self.