Feeding cats in the same household different diets is a logistical nightmare (been there, done that!) so sympathies!
Royal Canin's stance is "only feed them to your pet if your vet has recommended them", now this doesn't mean that your healthy cat would automatically be in for harm if they ate the prescription diet, but there would likely be problems long term if this was the only food the healthy cat had access to.
The dietary adaptions for cats with Chronic Kidney Disease (CKD) primarily involve reducing protein intake (in order to reduce the load on the kidneys), however cats typically need quite a lot of protein in their diet (for lean muscle mass, coat, etc). specialized foods such as the Royal Canin Renal varieties try and mitigate the reduced protein levels by using protein sources that aren't too hard on the kidneys but still they ultimately have a significantly lower protein content then a "normal" complete cat food. (RC Renal has a ~23% protein content vs the 33-40% found in their non-renal care foods)
So if his sister steals the odd nibble at his "special" food it's not going to be a problem - as long as she is getting adequate protein through her own food. Him stealing some of her's is more likely to cause problems though.