I forgot the whole plot and all the details are pretty vague here. sorry, but there was a fragment where the main characters brought police to the place where a carrashed long long time ago (or maybe not). There was a hidden dead body of a male child (the original heir of a big rich family) digged out of the ground near the road - and that's true. Maybe the child was even buried alive by his fafher - not sure again. But the big tragedy happened or was planned just because the dad had had a second family somewhere and wanted to give them everything (a new wife and a son apeared a bit later) but the protaganists exposed all his deeds (not sure how at all). Any thoughts? I read it in pre-covid times, and my brains worked better those days
There Will Always Be Protagonists With Delusions of Starting a Harem Arc 5 - He's irreplaceable, not a replacement (Ch. 52-61)
It sounds a lot like 'There Will Always Be Protagonists With Delusions of Starting a Harem' - Arc 5. Here the MC is the real son's cousin (their mothers were sisters). And he was taken by real son's father to pretend to be real son in front of mother.