From a female perspective I somehow think this is fine. If my older brother was jerking off next to me to show it's alright, I'd probably have a totally different take.
Is it just me or isn't the fact that the little sister being a minor, a bigger issue? I mean, if they were both adults, I wouldn't care so much especially when the little sister was only shocked and that was it.
I know this manga isn't supposed to be taken seriously but there needs to be a tagging system for warnings, aside from the genres. Or at least, warnings that can be found somewhere in the manga page.