They announced it to the other clan leaders and there goes to everyone. Miyuki is the future head, not yet the leader. (If I remember correctly this is what happened in the light novel.)
Where can I find this light novel and what chapter or page is it on?
They will announce it by sending a letter to ten masters clan and hundred families that Miyuki will be the next head and well, Tatsuya as her ***
Volume 16 and around last pages of the novel.
Can I get the link to it?
You can look for it in mahouka wikia
I think there's also a Reddit post where you can find the novels
I wonder if they will announce to the world of the new leader of the clan and then eveybody finds out that Miyuki and Tatsuya are of the Yotsuba clan?