Being the first person told is a form of trust, not permission to become the spokesperson.

Hearing news first can feel like a role, and the role seems to include telling everyone else. It rarely does. Someone chose you because you are safe to say it to before it is finished, when they may still change their mind about who else should know and when. The timing was theirs to choose.

If someone tells you something before others hear it, ask directly who else knows. Then say nothing until they have. Being trusted is the whole reward, and it is a larger one than being the source.