Because the mother sheep had horns? Depending upon the breed and genetics it’s not unusual for female sheep (known as ewes to non-progressives) to have horns.
https://www.sacrewell.org.uk/news/why-do-some-sheep-have-horns-and-others-dont/
In some breeds, neither ewes nor rams have horns. Same with cattle. Horns are not definitive characteristic of sex.
The mother sheep didn’t have horns in this book. There were both mother and father in the illustration with the kids. This is really the “super-sleuth” really taking things out of context. Didn’t show both pages of the illustration, just the one page. Liars gotta lie.