My personal opinion is that it's Bella, I feel like she makes the most sense. Like I've said in the past she's conveniently placed herself in a place where no matter who receives the money she'll have some access to it. Making herself the "Girlfriend" to the MC, and moving in with Elaine, being the caring and loving sister, even quitting her job to do so. And of course if she just gets the inheritance.
The only other option I can think of is Jolina, just because she has access to the computer in the Cafe. And my initial gut reaction whenever she freaked out when you first ask her out made me think she was banging your dad, but then she said that was because she thought you were on date? So that suspicion seems flimsy now.
With someone like Zarah I could definitely see her having an affair with the dad, she seems like a very easy person to seduce, and doesn't seem to have that much restraint. But I'd be pretty surprised if it was her? But also very happy because she's literally the only girl I have zero interest in.