Hi I'd like to help you with your question.
Your step daughter's child is reacting to the trauma of her father's death by regressing. She is going back to a behavior left behind when she was younger. Since she cannot voice how she feels about her loss, the bedwetting is a way of expressing the trauma she feels about her father's death.
How her mom reacts to her bedwetting is going to impact how her daughter processes this problem. If her mom acts out and gets more upset, then the girl will feel more traumatized and may continue the bedwetting. So making less of the issue is the best approach.
What your step daughter can do is reassure her daughter that she is ok. Also, she may want to talk to her daughter about how she feels. The child may be experiencing fears such as the loss of her mother, nightmares, reoccuring thoughts about her father's death, and feeling abandoned. Talking to the child helps her express her feelings and get reassurance that she is ok. This alone may help the bedwetting.
Therapy is also an important part of recovering. At the young age of 6, the child has no ability to fully comprehend why she feels the way she does and how to react to it. A therapist can help her either through talk and/or play to understand in a child's terms how to process her loss. To find a therapist, talk to the child's pediatrician. Or you can contact your local children's hospital for recommendations.
There are several methods that can help reduce or eliminate bedwetting. First is giving the child an outlet by talking to them and reassuring them. Two, make sure they are limiting liquids before bed and using the bathroom before hand. Also, talk to her before she goes to bed about staying dry. Make it light and fun, not a serious conversation. That way, she does not feel blamed or upset. Wake her up in the night to use the bathroom. Also, use liners under the sheets or mattress pads so she does not know they are there. Further embarrassing her about the issue will cause her to be even more traumatized and the issue may continue.
Here are some resources to help you further:
I hope this has helped you,