Answer:
Aunt Harriet did not kill herself or the baby, though it does seem like it at first. If you read the pages carefully, Joseph Strorm kills her actually. In the pages after Harriet leaves, David's mom says she will pray for gods forgiveness for Harriet, but J.strorm talks about the heresies of women, how they can get away with it but he says how aunt Harriet won't get away with it this time. The page after, David says how they found aunt Harriet's body in a river, his dad mentioned her in his prayers but never again. This implies murder.