"Get up lazy bones" - The author uses this phase both with the stepmother as a speaker and with the witch, thereby insinuating that the stepmother is the witch. This was common in coming of age fairy tale because of the jealousy and competition between mothers and daughters; the daughters are becoming beautiful and the mothers are getting old. Since it is not socially acceptable to hate your mother, the mothers were changed to stepmothers by many fairy tale writers. See the Grimm Brothers' Biography.
The stepmother takes them deep into the forest - she knows the place well because she is intended to be the witch. The witch's house is very deep in the forest, therefore it would make sense that the stepmother would know how to navigate through the forest well.
In addition, the stepmother dies at around the same point in the plot as when the witch dies. This is another incident which might lead the reader to believe the stepmother is the witch.