Child predator?

A few months ago, my sister, 16 years old, reached out to me and my other sisters to share some disturbing information. Apparently, my stepbrother (35 yo), has been sending her inappropriate text messages. This man has been my stepbrother since 2002, we were all kids and my youngest sister hadn't been born yet. My youngest sister is my mom's child, and my stepbrother is my dad's current wife's son, no blood relation at all. However, we were all raised together and treated each other as siblings, including my youngest sister because she was always around me and my other sisters growing up. So we are all "family" in a way. I was shocked to say the least when I learned and read the messages he was sending to my little sister. He is a history teacher, and she was in his class a few times a few semesters ago at school and I'm assuming that's where they got to experience each other on a daily basis. I thought all was friendly and family fun, until I read the messages he was sending her. He'd say how attractive she was, asking her if he came by to pick her up, would she get in the car, and very inappropriate flirty texts. She shared this with us because they made her feel uncomfortable. I reached out to my stepbrother and warned him if he ever spoke to her again, I'd turn him into his school as a child predator. He apologized heavily, understanding that what he was doing was wrong and I haven't heard from it since. He said he thought of her like a sister, but you don't flirt with a sister like that. A lot of things have happened in my own childhood that I am completely against any type of sexual or flirty feelings where we keep it on the hush or keep it as a secret. That's how my grandparents were able to abuse me and my sisters so well. So I spoke up and reached out to everyone in our close family so they can be on the lookout for my sister, and watch for anything from my stepbrother, and make them aware of what he is capable of doing, with my sister and the kids he teaches at the high school. I also reached out to my dad as my stepbrother still lives at home with my dad and his mom. My dad has never mentioned it since. Time goes on and COVID happens, etc. We recently had a baby gender reveal party and I wasn't sure if my stepbrother was going to be there or not, so I reminded everyone to just be aware. He wasn't there, however, my mom and my stepmother had a weird interaction that upset my mom, and she wondered why. I told her it was probably because of the thing with her son and my sister, which reminded my mom that no one had even talked to her about that. My mom still speaks with my dad on an occasional basis, and she asked him if him and his wife have spoken about it. My dad told my mom that he did not tell his wife and will not, which, of course, upset my mom and my sisters because we thought that was a really bad decision on his part. Not only was he not speaking about it, he was hiding it from his wife. So this morning, in my sleepless insomnia, I decided to reach out to my stepmother, letting her know my concerns. I thought about what I had said to him, and although back in the day that might have been enough, I feel like in this day and age, a better way to handle that behavior may be to report him, or get him professional help. I reached out to my stepmother, against my dad's wishes to keep it a secret. It's been a few hours, I'll call her tomorrow if she doesn't respond today, but my question is did I do the right thing? How would you have approached it? Don't forget that my stepbrother I consider as my brother, well I did until recently, but there is still a lot of family love, but he hurt my youngest sister, which ultimately hurt me. My stepmother and I have never been close. My dad likes to keep secrets. I'm not one for those, especially when I have a baby girl on the way who may have a predator as a step uncle. What does one do in this situation?

I live over 3000 miles away from my family. My concern isn't for my child, but for my sister and my family, and the children he teaches. Would you report him or handle it in the family?

In response to some comments below, this is me advocating for my sister. She came to us for help, we did do something about it for her. She's not traumatized and I ask her how she feels about it and how we handled it. It concerns me that he did this to my sister, not his stepsister, and I am concerned about the other girls he could do this to, which is why I'm reaching out. I'm well aware of what it feels like to be abused and nothing happens to the abusers, as I mentioned above with my own experiences. Thanks to those of you who have responded justly.