In game, no it's not really a cuck. In the code, it kinda is. Because of the hidden stat that gets forced on you for watching the scene. It really should have an alternate scene for the Alpha Path, or a way of influencing the stat change.
Oh and I agree about the Mom and Sis scenes should both be watchable. But the way they were written makes it impossible because it takes into account the Beta scenes. You either watch the Mom's mental undressing, and then Weico decides to call Marcus down or you watch Marcus making out with a topless Sunny. They both would end at about the same time.
It's easier to imagine the Alpha scenes both being watchable, because the Sunny scene is over in like a minute's time.
This is what I suggested to. Either the Self Confident Mom, or the Son as the "Hero who saves her from the uncomfortable conversation" interruption.