That's literally another reason not to do it at your parents home. Since the brother always seem calm and collected.
Lust trumps 'calm and collected'.
The MC is certainly laconic in most relationships, but when it comes to his attraction to his sister, he definitely has strong emotions, which he had before (barely) kept in check.
I always figured that once the taboo barrier was breached they would start emulating rabbits in heat. Reason gets left in the lust.
Without a doubt, having sex in their parents' home is at least risky. But things are going on between those two that defy rational thought processes. Isn't the whole incest situation a gamble?
I also hardly think it's the first time that two people (who weren't supposed to) had sex in a parent's house. That is a frequent real-life occurrence.