Have you been able to talk about this with a therapist? Are you trying to break the ice here before you try to do that?

If you are seeing a therapist, which you should be by the way, the best way to break the ice is to discuss the topic first. You can do that by telling your therapist you read about that challenge in an online support group and ask the T about his/her thoughts on it.

Believe it or not it happens a lot more than people realize. Children find themselves and they might explore with other children, but not really understanding what it really means.
It is also not unusual for parents to be totally ignorant about how that can happen too.

It also tends to bother someone even more when they have their own children too. It can become especially challenging when one's own child turns the same age as the parent who suffered that kind of scenario was.

There are therapists that hear about this kind of situation a lot more than you realize. It tends to be a more common skeleton than you think. Human beings are "sexual beings" and children discover this at surprisingly young ages. It is also not uncommon for the female child to give in as females in nature do that too. It is just that young females don't really understand what it means, not in any kind of "adult" way.

The history you are discribing is one where you had to step up and take on the parenting role which was not fair to you at all, what child really understands what "parenting" really means, no, children only want to "please" and see things "safe". That is what you did, you tried to make things "safe" in your own family and they still want that from you.

Well, you have your own family now and you "are' responsible for that and "only" that.
You should not bring your brother into that environment either. It upsets your own boundaries and can potentially be a threat to your child's boundaries too. You should set strong boundaries with your brother too, tell him straight out that you would prefer he not move into your area. You "can" be direct with him about how your past is something you would like to distance from completely and you wish him well in "his" life but you want to and deserve to have your "own" life now. It is high time your brother face that what he did was "wrong" too, and it is high time he deal that issue himself too.

There is this societal "loyalty" to family message that takes place, well, not if the family is dysfunctional. It is not your job and never was your responsibility to parent over your family. You need to break them of expecting that from you too. I am sure you are a very nice person and "caring", I hear that in you, but you really have to work on boundaries now as you will be also teaching your own child about that as well.

I am very sorry that happened to you, it was not fair to you and please don't self punish for it. It's ok to learn from it, know it is wrong and make sure your own child understands body boundaries and that they understand it also as something that not right even with other children.

I am sorry that as a child no one was there to help you set boundaries. It time you do that for yourself now, you deserve to have boundaries.