It's his name that's also his title. 'Shiou' is his name, and you can read it as Shi-death/corpse and ou-prince/king. (This is my inference based on my minimal Japanese knowledge. I have zero Kanji knowledge, so I could be wrong. )
Even with the dumb premise and even dumber MC, I'd have read this if it were just Rui as the FMC. But since it's a fucking harem on top of everything else, I'm out
I think she means she would have struck her son had he done something similar to the other prince.. The translation a d proof reading on this is a bit whacky.
The shitty development aside how is this guy so short when his sister is that tall. Clearly, he has the genes for a good height. Was he mal nourished as a kid that inhibited his growth??