Ian could not sleep, too worried about how Mina would react when she woke because when she thanked him at the hospital, he thought it was a good sign, but hours later, she treated him as coldly as when she first came back. Now that she 'thanked' him while lying naked in his arms, he was worried that her words may have been alluding to something else entirely. Ian did not know why his mind went that far, but he was terrified of losing the woman; so much so that he was willing to lie to make her his.
Sadie had in fact come into his office telling him that Mina would hurt herself, but Ian had taken her words and twisted them for his own use. He told Mina that her sister begged him not to say anything and he told her that the only reason he did was because he wanted her to reassure her family. His reason: to prevent 'someone' from using it against them.
