Personally I don't think a sister is needed... You now have 6 beauties to partake the pleasures with (Julia, Cassandra, Lori, Barbara, Daphne, and Haley). Any more and time with the primary beauties (Julia, Cass, and Barbara [IMO]) becomes less. I'd rather focus on just a few rather than add more. Just my opinion...
By the by, what do you think is going on with MC's dreams? Cass' reaction to his dream plus her having woken up from a weird dream of her own makes me think we have some psychic link shit going on. Like she was having the same beach dream but woke up before the shark stuff.
The dream stuff has been a weird factor from the start.
You beat me to it, I was going to ask something extremely similar. I don't think it's a psychic thing, I think the house is bugged with mini speakers that allows for subliminal messaging to be broadcast that induce dreams along the lines of whatever the programmer desires. I think the phantom woman isn't so much someone he had sex with, but someone who was in his life and is someone he desired... Maybe a former Nanny that worked for the family while Cass was pursuing her career, or one of his father's former secretaries... It's too early to really tell, but it does bring a unique and interesting perspective to the story (at least in my experience). It may even have something to do with Daphne, based on an early past scene (around .6 is when I first came in to the game and it was there). Maybe she's trying to corrupt Cass still and using the MC to accomplish her goals... So many possibilities, all great hooks to keep you tied to the story and await the next update.