how would you feel if your mom started getting corrupted by some playboy and old fuckers? To the point where she is turned into a whore and starts to disregard your well being completely just to have more cock?But why does MC care about his mom cheating on his dad? Shouldn't his dad worry about that while the MC worries about his girlfriend or lack of such instead?
In that sense, your relationship with your mom is "stolen." That's the gist of the mom/sis NTR. There can be some layer of love/lust towards the mother on top of this too, but it's not necessary.