I agree completely, there's a difference between legit BDSM and this. I don't see the safe-word, aftercare and informed prior consent anywhere.
Oh, I forgot Translator-sama does say something to this effect in the intro to chap. 3. He/she did her best in trying to warn us, but it's impossible not to react nonetheless... Translator-sama herself calls it "a twisted ride".
I also think the one who keeps glamorizing it is that darn cousin of them. Call it an excuse not to do something to actually help them (and he was supposed to be one of the "sane" people in the story). It's actually him, I think, that is the most guilty one here, especially if it ends bad (with a sad ending) like the translator hinted.
I've read some pretty disturbing shit in my day that involved rape, incest, abuse, etc. But this manga really rubs me the wrong way for some reason. It's almost like... they're trying to glamorize physical assault? I find myself not really understanding or sympathizing with their current situation. They came from a toxic family where the father abused the mother, so the boys run away when the mother died.... And then "big bro" decides to start the circle of abuse all over again with his brother and calling it "sadism"???
Personally, I think calling them a sadist and masochist is completely inaccurate. Where I come from I call it: "beating the living shit out of your emotionally damaged sibling and sodomizing them".