What is the blank that Silas Marner talks about that he must fill in chapter 11 and 12, and how do his neighbors help?

Silas needs to discover the orgin of the baby in his home.... how she came to be there.

Could this be his little sister come back to him in a dream—his little sister whom he had carried about in his arms for a year before she died, when he was a small boy without shoes or stockings? That was the first thought that darted across Silas’s blank wonderment.