The Man I Married Died Five Years Ago

My husband said he had to work overtime on Memorial Day, so I went to visit my mother-in-law's grave alone.

When I arrived, I found a woman there with a small child, paying her respects at the very same headstone.

I asked who she was, and she answered as if it were the most natural thing in the world:

"My husband was too busy with work today, so I brought our son to visit his grandmother's grave."

I froze.

My mother-in-law only had one son. If this woman was her daughter-in-law, then who was I?