Who told Toko that she was adopted?
Nobody ever told Toko directly that she was adopted. As you mentioned, she overheard her relatives speaking about it, and she confirmed it by looking it up in a registry.

Quoting a line from Toko's article on wikia:


  She found out the truth later after listening in to some of her relatives' conversations, and confirming it in a registry


So Suguru just got angry and there was no specific person who revealed the secret