
Can someone explain to me, why his family like that, they sent him to be a sacrifice but why are they interfering

The principal branch of the family is the people in power, something like the principal clan. From what I understand, they control the other smaller clans and force them to give up the sons lord Mugo chooses as sacrifice. As compensation they make sure the family is not poor, but if the sacrifice refuses, then the principal clan will murder their little clan.
By his look at the end, i don't think he is the first man jerking him off