Are you new to her household, or have you been married to her mother for a lot of her life?
If you living in the household is new, and these behaviors are also new (don't know that they are) this may be her attempt to try to assert control in an environment she has no other way to get control of.
If her mother is having frequent panic attacks, this child is probably feeling extremely insecure. Does your wife take medication or any other therapy to control her panic?
Also, when you say "more often than not" that she soils her pants instead of using the toilet, do you mean 3 - 4 changes of clothing at school?
I expect you are dealing with anxiety - panic attacks are examples of anxiety and anxiety is inherited. I doubt if behavioural has that much to do with what you are seeing. By the way, many children who suffer from anxiety have problems using the toilets at school so I doubt if the reply "because I was being lazy" is correct. She probably is too "afraid to go" although I doubt if she is aware of this. I also suspect, in addition to her toileting and sleeping issues, she has eating issues, perhaps social issues and displays tantrums and frustrations. By the way, the ailments she feigns to suffer when going to bed probably are "real" to her - anxiety has physical manifestations as well as mental and emotional.
It probably is time for her mother or father to take her to the family doctor to discuss how best to help this child. If this physician is not experienced in anxiety issues, then they should request a referral to a specialist with experience in this venue. I wish you the best ,,,,
What relationship do you have with this child?