I only got my diagnosis this year, but when I got an anecdotal diagnosis years ago, I went down a rabbit hole of research to understand it as much as I could, and I've noticed that there are quite a few kids and adults with it, although it does show up differently in girls than boys to an extent.

I don't think it's overdiagnosed, it's just the fact that it's better understood these days and there are better ways of identifying it (i.e. if a close family member has it, child may have inherited it, the new eye test). Your sibling might have indeed had a milder form of autism, but due to it not presenting the way they expected it to, they were not diagnosed. However, there is still an overabundance of self-diagnosers and people who've gone on webMD and thought 'ive got a few of those symptoms, I must have it'. The whole issue with any mental disorders is that it's pervasive and actively impacts a person's life in a negative way. If a person has minor symptoms but their life isn't negatively impacted or requires workarounds for it, it's not a disability.