@Carycy - I don't think it is so much labelling every child, but this seems like a regular occurence for the OP's DN. If any of my DC had been regularly soiling themselves by the time it came to starting school, I would definitely have sought the advice of the GP. It may just be that this particular child is a late developer but her parents really should seek advice to rule out any other cause.
@Playnoh - why do you blame the childminder? I used to be a childminder and have had times when pre-school children have had accidents. It takes quite some time to thoroughly clean a child that has soiled their knickers/pants and clothes. Bearing in mind, this is wraparound care, the childminder is probably busy enough after school, sorting snacks and possibly tea, along with supervising homework, entertaining young children etc. The childminder may be incredibly fond of the OP's DN but cannot feasibly cope with having her soil herself most days. Imagine you are preparing a meal for five children after school. You are committed to getting the children fed before they are collected by their parents. Halfway through cooking tea, you discover a child has soiled themselves. It is probably going to take you at least 10 minutes to completely sort the child out. You either have to leave tea cooking, with no supervision of the other children or the meal. Or you have to turn the oven etc off, delaying tea. Let's be fair here, however much the childminder is fond of her charges, she is, ultimately, trying to earn a living and if she feels this child is causing issues for the care everyone else gets, then she may have had to make that difficult decision to not look after her any longer.
I truly believe that OP should strongly advise her sister to get her daughter checked by her GP. It's a starting point - if it is down to maturity and development or laziness, then they can advise on how best to manage the problem and if it is that there is something else going on, the sooner they identify a problem, the sooner it can be fixed.