A few thoughts, that others are extremely welcome to weigh in on. I am by no means an expert, but shall attempt to view this calmly and rationally.
First of all, the messages you sent to your friend do in no way sufficiently detail the issue, not that I necessarily want further details. Based on eight vague messages I have no way of determining whether or not active, calculated grooming has taken place. Based on the messages you seem to have been unaware of implications at the time, which I've taken to mean that in hindsight, perhaps some of that behaviour was inappropriate. I have no way of knowing that. But "grooming" is a specific thing, with a specific meaning, not a word to throw around lightly, so here's what I do know.
Talking about sexuality with a younger sibling is not abuse. They might have questions that you, as an older sibling, might be equipped to answer, or just be curious. Being honest and having open communication about those topics is a perfectly normal thing. It's not grooming. I don't know the details, and I don't want to know them, but there's nothing inherently wrong with the topic.
Letting a younger sibling listen to implicit song lyrics is also not abuse. You could just be wanting to show off a cool artist or a good song that you like. You even appeared to wait until they were older to show them a particular song, indicating that you deemed that one inappropriate. That is not abuser behaviour. Groomers have no concept of inappropriate, and they don't care if they make someone uncomfortable. The intention of a groomer is to constantly push boundaries and get closer to their intended victim. A groomer is aware of what they're doing. You can't groom someone by accident.
I am not asking for more details, but I am asking you to look within yourself and see if there was an intention of hurt and manipulation. The PPC Posting Board can't decide that for you. We are not your judge, jury or executioner based on eight messages where you proclaim yourself an abuser and not accepting arguments against it.
You came to the Board for confession, accepting whatever punishment we decided on, but I for one am not going to sign off on any punishment here. What I am going to do is to ask you, with every fiber of my being, is to please talk to a therapist. You have a lot on your mind, and a conscience that's weighing you down, and you seem in desperate need of a professional who can help you figure out all your thoughts.
You seem very worried that you are a bad person. For what it's worth, abusers don't usually worry so much about things like that.