Your screen name says it all. So sorry you had to come here but glad you found it.

Here is a place to start to find out the laws in your state: https://apps.rainn.org/policy/?&_ga=2.129921931.647267384.1627239066-458265023.1627239066#report-generator There should be a banner at the top with the 24/7 national hotline of the rape, abuse, incest national network. They are a WONDERUFL resource, you are welcome to call as a parent and even though it has been years since it happened. It is totally confidential, they don't track your phone number and actually they are not mandatory reporters. (I just found that out when I called them myself last week.) If the person who answers doesn't know the laws and procedures where you live, they can put you in contact with someone who does. And they offer all sorts of support--emotional, legal, practical, etc.

My quick off-the-top answers to your questions: 1) Generally mandatory reporting only kicks in if the one who was violated is still a minor. But check with the resources above to be sure. 2) Ultimately, the one who would make the decision whether or not to charge is the prosecuting attorney for the state (DA, States Attorney, whatever they call it where you live). However, many will take the victim's wishes into consideration. There are a lot of unknowns when the offense happened when the offender was a minor but is now an adult. Rainn.org might be some help there but the person who would know best is a defense attorney in your state. I have no idea what your daughter wants, and at age 18 the ultimate decision of whether report will be in her hands. But it might be good for her to have an idea ahead of time what is likely to happen after she reports. It could be anything from the police saying, "Thanks for the info but there's not much we can do" to your son ending up in prison and on the sex offender registry for life. I hate to be so blunt but that is the reality, both ways. I have no idea what your daughter even wants to see happen at this point.

Some other good resources:
mosac.net
Book or e-book on Sibling Sexual Abuse: https://www.amazon.com/Sibling-Sexual-Abuse-Confronting-Americas/dp/B08RH2C52C/ref=sr_1_3?dchild=1&keywords=sibling+sexual+abuse&qid=1627239871&sr=8-3
https://www.kkccares.org/resources/sibling-sexual-abuse
stopitnow.org

Come back any time, you don't have to go through this valley alone.

The last two lines below the book reference are separate links, FYI. It didn't format well.

How are you doing OP? Thinking about you, remembering what a horror those first days and weeks are.

Horror is a good way to describe it. My daughter had her first counseling session. The therapist is very gentle. She didn't ask for details just yet. Did warn us that she may have to make a report when the time comes. So... waiting for the next bomb to drop.
I have read the book that's in the link. Thank you.

I'm sorry to hear about your all too familiar situation. It is up to your daughter to decide if she wants to report her brother. If (when) she does, it's likely that the police will interview you by phone once the charges are filed. Then they will seek out your son for an interview. If he does not respond, it's likely that they will not pursue prosecution. It often ends up "she said, he said." Only if he admits to any wrong doing would he be investigated and likely charged. It's a difficult situation. If your son decides to talk, he should not do so without a lawyer sitting next to him. That is my advice. Your son is 24 and an adult and needs to make this decision on his own. He can research the prosecution statistics for situations similar to his. Right now, your daughter needs to seek therapy. Show her as much love, care and support as you can. She will have to heal in her own way and in her own time.

That is great that she has met with a therapist. I hope the therapist can navigate this so that your daughter will be able to make the decision of whether or not to report and if so when. I remember my daughter's therapist saying that when she works with teens, who in her case were under age 18 and she would have to make a mandatory report if they told details about abuse, she would let the teen know, "I think you're getting close to something I would have to report." She did it to maintain the teen's trust and also to let them retain as much control as possible--since loss of control was such a traumatizing part of their original experience.

This is covered in the mosac.net site and book, but I also recommend starting to look for a therapist for yourself. This is going to be a long road no matter where it ends up going. Just having someone that YOU can talk to confidentially, a place to process the feelings you might have to keep a lid on in front of your kids, is really important.

If you feel comfortable sharing yet--How was your relationship with your daughter leading up to this? Is she living with you or near you? Was she showing any other mental health issues before this? How about your son--how was your relationship with him, does he live nearby? Do your son and daughter have any contact with each other usually now? i.e. is it easy to keep them apart while she goes through the therapy process? That might be an important thing for now. Even if your daughter isn't worried that he will violate her again, even if they have been together and seemed OK in the years since it happened, the process of disclosing and therapy can bring the memories really close and cause a lot of trigger reactions. That's something I didn't understand when I first found out.

If you message me you want it (the envelope at the top right), I can send you a file with info on the pros and cons of reporting and what to expect.