Thank you so much for writing. I understand why you are concerned. There is so much in the news lately about the abuse of children by people they trust.
I reassure you: Children generally experiment with romance and sexuality. Little ones pretend to be princes and princesses who “marry”. Children from 2-6 years old to do things like look at or touch each other’s genitals or to try to see adults naked. The American Academy of Pediatrics (AAP) states that this is normal. Young children are exploring their bodies and trying to figure out what all the mystery that surrounds privacy and nudity is about. Red flags go up when there is more than 4 years difference in age and/or when kids try to do something that mimics overt adult sexual behavior.
In your case, I wouldn’t identify your experience as ” overtly sexual”. Let’s review what you said: You were curious about what you were seeing on screen and tried what you saw for about 5 seconds. When your brother asked you to stop, you did. You immediately understood that you had crossed some boundary. You never did it again. That isn’t abuse. At age 10, you were curious about what adults find so interesting about kissing and you tried it out with someone you felt safe with. But you were already aware, maybe not quite consciously, that there are “rules” about who it’s permissible to kiss on the mouth. You had already somehow absorbed that cultural taboo.
I’m most interested in what you didn’t do: You didn’t pressure your brother. You didn’t use your size and the fact you were older to force him nor did you manipulate him. You didn’t get pleasure from pushing a boundary and you didn’t repeat the behavior. That tells me that your behavior was merely experimental and not intended to be abusive. Those are the things that abusers do.
I wouldn’t worry about your brother’s behavior with his girlfriend. He is well within the range of normal. Kids that age are just beginning to pair off. Sometimes claiming to have a boyfriend or girlfriend but keeping it platonic is a way to avoid peer pressure to be in a relationship when they aren’t quite ready. He is also beginning to explore his own sexuality which includes asking the question about who he is most attracted to. Such questioning usually goes on for a number of years and is perfectly normal.
I think remembering the childhood incident is a reflection of your own developing moral code and your sisterly love for your brother. I do think that sharing your concern right now is more for your own benefit than your brother’s so do consider what you hope to gain from it. If it is to only make yourself feel better about it, I think you should leave him out of it. Just give yourself credit that, at only 10, you understood that kissing your brother wasn’t appropriate.
If you are concerned that he is worried about it — and since you have such a strong sibling bond — you could bring it up as something you regret but that you understand now that at ages 10 and 4, you were copying what you were seeing, not doing something incestuous.
I wish you well.