I am going to run through the game again and confirm, but I don't think he tells anyone he thought Stacy might be his sister. He does let Jamie know there are things he is still trying to figure out about her. Now it is possible that she found that information out from Jamie or one of the other girls. Since we only see the game through the MC perspective there is a lot of possibilities, either that or this is part of the week one re-work that has not be implemented yet or maybe it was and we missed it.
I have a lot more to say on topics, but again I am waiting for the normal stupid topics to die down again before I toss out my full thoughts and theories about day 8 part 2.