I agree it doesn't sounds great, but I'm not sure what you actually KNOW.
You thought he was wet when you dropped him off. He was very wet when he was collected. Are you certain his nappy was never changed? Are you certain his clothes had been soaked through for hours?
9 kids seems excessive, I agree. Are you certain she was the sole carer on duty?
I don't think the questions about whether he can roll or sit to be that unusual. Some kids are advanced. Its fair that she should ask this. Same about bottle warming.
If you can be certain of the other things, then I'd make a complaint. If you're not certain, I'd enquire with the carer.
Re the sole caregiver - is it possible that one was away for a few minutes in order to drop off or pick up out-of-school-care kids? (By that, I mean kids who attend public school either half days or full days, and someone has to ferry them to or from school). Even then, I'll agree that 9 preschool kids seems to be too many, but there's so much that is not known in this situation.
I will agree with a PP that a SIDS situation that was thoroughly investigated should never be a black-mark on the record of a reputable daycare. While very rare, it can happen just as easily at a daycare as it could in the family home.
The key phrase here is reputable
I am a competant, security cleared, first-aid-trained childcare provider. I have a toddler of my own and care for one other toddler. I could, conceivably, take a second or third child, depending on their ages. However, I would find that one or two more children in addition to my own toddler would severely tax my ability to provide competant care.
Every place is different, but most states have rules governing unlicensed care, as well as licensed care.
It is important to note whether any of the children belonged to the care provider, their ages, and the general state of the house.
What the OP described would have set me on edge, too. I had a sitter put LK to bed the other night without changing her diaper. When I went in to get her in the morning, she was soaked through her diaper, through her pajamas, and through her sheets. Mr K thought that the sitter had just put LK in the wrong diaper (she has specific nighttime diapers because she's a heavy wetter), but that bothered me, too, as I'd given specific instructions about needing to use the nighttime diapers, had put the daytime diapers away, and laid a nighttime diaper on top of LK's pajamas. Mr K checked the nanny cam and discovered that the sitter had not, in fact, changed LK's diaper. It's not a huge deal, but I suspect she didn't follow certain other instructions (LK's toothbrush was dry). I simply won't be using that sitter again.
Failure to follow simple instructions is not a sign of a competant or reputable caregiver.
The OP left her child with instructions that he was wet and needed to be changed, and a change of clothes in case the ones he had on were soiled.
The child was given back with an even wetter diaper and soaking wet clothes.
Based on the wetness of the diaper and the state of the clothes, the OP determined that he had not been changed as instructed. This is a reasonable assumption. She further assessed that it's likely her child wasn't changed because the caregiver was overtaxexd. Again, this is a pretty reasonable assessment.
Unfortunately, after re-reading this and re-reading the OP's actual question, I doubt anything would come of a complaint. I think the OP should file one anyway, just to have it on record. It's possible that it may help someone else in the future.
LilacGirl, stick to the facts - what you and your sister in law directly observed (ie 9 children under the care of a single provider, that your child was left in a soiled diaper for presumably for the entire duration, whether there was any rash or redness from being left in wet clothes - this is an indicator of how long he was left wet). It would probably also be helpful for your sister in law to write a note on what she directly observed to include with your complaint.
A helpful tip for the future: Get a different kind of diaper for any new caregiver to use. I use a different kind of diaper with LK when I send her off to her Sunday School class, and will be doing the same with babysitters inthe future. This was I can see at a glance whether she has been changed.