Normally I do not like NTR bit I think in some cases, it can be used for the story. (which is rarely done) The problem I see is, that you have to decide at a certain point, if you want NTR in the game or not and that decision dictates the rest of the game.
My optimal solution in this case would look something like this:
The mafia boss is starting something with your mom and you now have to decide. Do you want to step up or not? The decision to step up could be the starting point for the MC, to get a reasonable motivation. For example, he steps up but gets beaten by the mafia boss. Afterwards the mafia boss does something to your mom and you are forced to watch. Wether you like NTR or not, you now have a great motivation for the MC and a connection from the player (us) to the MC.
Afterwards you can go on with the game without of anyone laying a hand on your girl(s). But I think it is wrong to judge a game immediatelly after something happened, you don't like. If something like that happens, the reward later on when you finish the boss will feel so much better.
But that is just my opinion.