I like boys. It just scares me that this picture comes to mind when the topic of sex is brought up. Not only is it your only sexual experience, but it was your first, and you were too young to know what you were doing. Even if you had participated willingly, it's not unusual for young kids to experiment with other little kids, even of the same sex. However, in this instance, you were coached by your older sister, whom you probably trusted, or you may have felt intimidated and afraid of not doing what she said.
Bodies often respond to stimulus even when the mind is resistant.
This is a short term therapy used by licensed therapists which uses deliberate eye movement in conjunction with recollection to treat various forms of trauma. Even though this experience might not seem traumatic, it was. Those memories are 'trapped' in your mind and can be released and dealt with through this new non-invasive treatment. If you can't do that, it might help to talk to a counselor. You were abused and that needs to be talked out with a professional.