> But our heart is our sister-in-law.
A husband's sister is said to spy on her brother's wife during his absence from home. Reference is to a young wife who refused the advances of a young man, even though it was obvious that no sister-in-law was present to spy. Asked about her relationship with the young suitor, the wife replied: "But my heart is my sister-in-law." The saying may be applied more widely to any circumstance where a person is entrusted with a task without supervision.