Did a sin when i was 13 or 14

Assalamualaikum. Salamuaikum I do not know if you are responsible for this but I am desperate .. I’m a sister and Religious Alhamdulillah. I have a concern that has not let go of me for years. In my childhood (or maybe this already counts as youth) - I was 12 or 13 or 14 years old (don’t remember so good) - I was in bed with my brother, who was 2 or 3 years old at the time. out of curiosity, I touched his genitals with my genitals. even though i remember- that it was wrong but i did it .. then I let go of it and never did it again. 15 years have now passed.. but i read something in the internet about death penaltry = sharia law. I have regretted it, but I am deeply ashamed. I read on an English fatwa site on the Internet that looked very serious that there is only death for something like that in Islam and that you shouldn't pretend to be a believer in this world because you did something wrong and nothing helps you except that Punishment death penaltry in the world, so you should kill the person who did this. But I was very young .. I don't know if this can be excused- I regret it so much .. with all my heart and I disgust myself for doing something like that ... As I said, I don't know why I did that. Also to my brother ... I don't know if he knows anything about it, because he was quite small but I'm ashamed and .I don't know what to do because the news from the UK made me panic ... please help me. Because I'm really desperate, can I really not go on living? Does Allah really say there is such a punishment? Thanks for reading this. Wassalam.