So, when a conflicted dad decided to consult the moral compass of the internet otherwise known as Reddit's "Am I the As*hole" about his son's clueless comment toward his sister, people were quick to help deem a verdict.
My son and daughter (both in early teens) have a reasonably good relationship with each other (and me, I hope). A few days back, J had her period and accidentally stained her bedsheets. She was quite embarrassed.
Seeing the dirty sheets in the wash early (we normally change and wash all our sheets together), K was curious, and when I explained it to him, he said loudly (and in J's hearing) "oh my god, that's really disgusting." J hid in her room for the next few hours, and it was obvious at dinner she'd been crying.
I took some time to speak with each of my children alone. With J, I emphasized that periods were natural and, especially in early puberty, your flow can be pretty inconsistent. It was mostly just helping her come to the realisation that it wasn't "her fault."