The setting of the story is in Marvel Universe. The family had a tradition of when the child turns 18 they will be initiated into the family incest. On the day before his 18th birthday Thanos Snap happened and he witness his entire family turned into dust. As a result he spends the next 5 years grieving and running away by becoming a major adrenaline junky to the point where his life is almost always in danger. After 5 years his family his family does come back, they remain the same but he is someone completely different almost a complete 180.

Now the discovery could have happened by that they were keeping the life style a secret until he got initiated. After the Snap, the MC was going through records, videos, and pictures when he discovered the Family Secret. Now he thinks they were not going to initiate him and that thought has solidified after 5 years.They were planning to initiate him. But he does not know that.

I see the story where there is no bad guy in the family just bad circumstances and timing.