Look it isn't hard. The reality is that, personality shifts are way too irresponsible between the paths. You can make the argument that Marcus is a expert suducer and it's still too hard to swallow. The part you seem to be missing is that even as someone who is seducing others to a life of Depravity it takes time. Life isn't a porno and if you just want a porno go fucking watch a porno where all of that unrealistic shit happens. If your trying to tell a good story and not a prono script then sorry. Marcus can't bang Sunny super fast it breaks character. If she started off more on the slutty side things would be different. But her character was established. END OF STORY. Now Marcus should have to WORK for it. I'm not saying it should be easier for "Alpha" path to hook up with Sunny. I think it also needs to still be slower than with Marcus because the incest barrier would be more difficult to break BUT. She shouldn't go full on slut so quickly for Marcus because that simply isn't immersive.
You do make good points. Sunny probably did fall for Marcus too fast, based on her straight-arrow, school-first personality. After all, Weiku, who has far more direct influence over Tracy and far more experience than Marcus, hasn't got anywhere near as far with his target (Tracy).
On the other hand, it did take Marcus 20 full days working on her and the story does hint (actually it straight up tells us in the character pages) that the ladies Philip lives with have very strong sex drives. We are told they've been repressed until now and we saw the impact of that repression when Tracy fell into bed with Lucas right away after being loosened up by drink. Philip also observes during the Sunny-Marcus fuck in his house that "It's like there was this perfect slut hidden below the surface", another direct reference to their highly sexual nature. Maybe, just like beta Philip was unsettled by the move (the game's not called
Culture Shock for nothing), Sunny also kind of lost her character footing - 'Everyone in Hawai'i seems to ball, so maybe I should too to fit in', kind of thing.
Part of Marcus' speed may have also been due to the beta Philip's inaction. In a place where he might have played the good brother and helped his sister, he simply let Marcus continue to charm her. Before they moved and Philip felt more comfortable, he may have more actively helped Sunny stay pure.
However, all that said, I'm a fan of long, slow seductions and corruptions, so I would have been happy with Sunny-Marcus going slower too and it probably would have fit the story better, especially if as I said earlier, the story is only half over.
(Well, maybe not, because the 60-75 day release cycle for this game drives me crazy. Talking about this makes me want the next episode now.)