We don't see any reason to think he didn't tell Sarah. So why did Sarah laugh? Sarah was a tzaddika, and a prophet greater than Avraham, let's give her the credit she deserves.
She heard the bracha/prophecy, and she laughed because she heard something wrong with it. It sounded like it was temporary, so she laughed as if to say "you call that a bracha?". The malach respondes הֲיִפָּלֵ֥א מֵיְהֹוָ֖ה דָּבָ֑ר and reassures her, so her laughing was lobbying for her son. The malach said אָשׁ֥וּב אֵלֶ֛יךָ כָּעֵ֥ת חַיָּ֖ה וּלְשָׂרָ֥ה בֵֽן, which is actually a reference to after the akeida: same malach who told Avraham to put the knife down was the same one now giving the bracha! When Sarah got the news, our imma Sarah was able to die reassured that she had saved her son, by laughing and insisting on a complete bracha.
She, as with the Shunamit woman, were not saying "I don't believe you" through the laugh, they were both saying "I don't want something temporary". Avraham didn't pick up on it, because he was a seer prophet ("he raised his eyes and he saw"), Sarah was a hearer ("Sarah heard"). What did she hear? A bracha, to be permanent, must be face to face, and she spotted that the malach gave her the bracha, but to Avraham! וְהִנֵּה־בֵ֖ן לְשָׂרָ֣ה אִשְׁתֶּ֑ךָ Something sounded fishy about that...