I am wondering if you have said anything or asked your son about what your daughter said your son did? What is his mind set on what he did, why did he do that. I think it is important to find out where he is or what he is thinking before an assumtion is made. I'm just thinking the amount of information given to him at his age can be based on his mind set. What did he think he was doing, why did he do that, those kind of questions can quide you on the information you need to provide him at this point. I don't think he his to young to learn where babies come from or how they are made, but the topic of sex as a whole may not need to be addressed at this point. I think the other topic that needs to be talked about is privacy, respect, and inapropriate touchingand behavior. Explaining to him that boy and girls are made different and why is a great lead in to how babies are made. I think that your family unit needs to keep privavcy at the top of the list. All things that revolve arround dressing and undressing like shower and bath time, going to the bathroom and changing clothes needs to be done behind closed doors. And when the door is shut for privacy that needs to be respected. In other words brother does not need to see mom or sister w/o clothes on, he should not be allowed to because it's private. Nor should sister get to see brother. I also think it is important to impress upon both children that their bodies are theirs and they are private. No one gets to touch them or see them except a dr. or mom and if anyone tries to or wants to touch them, kiss them, see them (even a dr.) and they feel uncomfortable they have the right to say no and if the person continues or forces them they need to know it's ok to tell you or any adult. I have to say your daughter should be given a gold star (and so should you) for feeling comfortable enough to tell you what her brother did. I doubt her brother ment any harm he just needs to know it is not appropriate. Those boundries need to be set understood and not crossed before you give your son to much info. that could backfire. He may have some ideas, but before he gets all the facts he needs to know we don't get to touch anyone anyway we want when we want, and no means no that goes for him too. He gets to say no too. We are all sexual by nature and he knows what feels good, he did the very first time he reached down and grabbed himself during a diaper change. And that part falls under privacy as well. Kids play and explore with their own bodies and that needs to be done in private as well. I would guess he is probably getting some info. from friends as well. Because no matter how much you watch what he watches, games he plays, and how you or what you teach him at home there is no telling what goes on in the homes of his friends. So I would let him know he can ask you or talk to you about anything that comes up when he is with friends. It is a good idea to keep an eye on him and his sister together and what he sees or what is done arround him. I just think before you have a full blown sex talk you need to set the boudries and enforce the respect and privacy, talk about what is appropiate and that it always ok to say no and always ok to tell and talk about whatever. And also before you determine how much of the sex talk to have find out what he knows and where he is or what he thinks on the subject. And that is also important because you may need to correct a few things. No matter how much you determine is the right amount to talk about with him use the right names for stuff, but also clue him in to some of the silly names so he knows you know them. That part kid of breaks the ice and brings a few giggles. I had one mom tell me once she had an esier time talking with her kids about those kids of things back to back. She did say when it came to the serious stuff like privacy, repect and no means no she looked them straight in the face so they understood the importance of that part of the subject. Anther good tool for the sex part of the conversation is a book called Where did I Come from. It is all in cartoon pictures and silly looking to kind of make it ok to giggle a little. I just think he needs some information, and correct information and to know the subject is always open for discussion.
I wish you and your family all the best in the world and I also want to say I am so very sorry for your families loss. I know it can't be easy and you have my fullest respect for dealing with such a loss and continuing on as a great and involved mom.
Please keep us all posted on how things work out and what works.
Best wishes and stay well