My daughter just took puberty for this year's sex ed (grade 4). They deal with conception *not the sexual aspect but how babies are made from a sperm and an egg and how it develops* in grade 5.
In their school it's mandatory. BUT I think it's good especially since I started my TOM in grade 5 - and she will be too - she's already developing. I explained to my daughter before she started classes, that what she learns is not something that she should talk to with her younger brother or younger children. If she had any questions she could come and ask me or her father at any time. She said she doesn't feel comfortable talking with her dad lol.
Because of the "talk" she does not talk about it with her younger sibling.
I suggest doing the same

Explain to your son that his sister is too young to talk about it, and if he has any questions he can either talk to you or dad
fyi - I wouldnt be suprised if your son knows all the nitty gritty anyway - I know it sounds horrible - but kids talk. I overheard my daughter's friends talking about *sorry* oral and anal intercourse ("you cant get pregnant or STDs etc etc etc) last year and how all the kids in grade 5 and 6 are "doing it"- and she was only in grade 3. I had taken her aside and explained that it was not a very responsible thing to talk about and that actions like that can lead to a LOT of trouble until she understood completely what it means. She was really red in the face - but understood that it ment trouble. I had also explained to her at the time that if she has any questions - to ask me and to never feel embarassed about approaching me. Since then she asks and tells me anything.
I then spoke to the parents of the kids and the school. I was pretty embarassed myself, but I thought their parents needed to know what their children were talking about.