Her GP said that daily incontinence in an 8 yr old child is "normal"? Just because her parents arent around and her sibling is caretaking? I'm a foster parent and recently had an 8 yr old girl and i can tell you that she soiled herself twice the first couple of days i had her (on accident, due to some bowel issues) and i found THAT very concerning enough to mention it to the doctor. If she was soaking a pullup during the DAY and didnt seem to care or notice i would have her into the pediatrician ASAP and if that doc thought "oh thats normal dont worry about it" i'd be finding a new dr. I would either think something severe was wrong with the child medically (infection, bladder issues, etc) or that she needed therapy ASAP. Heck, i fostered her 4 yr old sister at the same time and if SHE had a similiar issue i'd be very concerned. I wouldnt be as concerned with nighttime wetting as bedwetting has a strong genetic component and kids can take years to fully nighttrain. Day wetting is a different issue.
I'm just not understanding dealing with daily wetting for such an older child for MONTHS and not yet visiting a therapist.
How often does she speak with her parents? Seems this could occur quite often via skype or something like that. Also have you discussed this issue with them? (I also noticed you refer to your sister's parents as "her parents" rather than "our parents" or "our mother and my stepdad" or "my dad and stepmom" or whatever the family configuration might be)