There are times to be very polite and times to be very frank. This is certainly the latter.

"While I appreciate the offer of a sleepover, DD has certain medical conditions that do not allow this to happen. If this situation changes, I will certainly let you know. But until then please do not ask again."

If they ask another time, just say "We talked about this in the past and nothing has changed".

Absolutely not. This isn't an issue of your ILs disregarding your preferences about sugar consumption, screen time and sleep schedules. Your daughter has serious food allergies and other medical issues, and your ILs don't I deratand these limitations or don't care. It sounds like your daughter could get very badly injured or worse. You are her parent and you have to stand up for her, and protect her, no matter how much drama it causes. Just say NO as politely and as firmly as you can, as often as needed.

Another "no way" here, and I think it could help to explain it to your DD too. My DD doesn't do sleepovers because of non-medical reasons (can't fall asleep, sleeps with the light on, comes to my bed during the night, etc.), and she completely agrees with me that she's not ready for a sleepover. Think of it as starting to teach your daughter to advocate for herself...