Had similar thoughts about Bella until i met Theresa. MC's father was a monumental asshole, but he wasn't stupid. Just look at the games he plays with them even from the grave. If he had a relationship with Bella this would've exploded in his face way too easy. They're both sisters and it wouldn't have been that hard to find out he was nailing them both.
So my main suspects are Theresa and .....
Jolina.
Theory behind Theresa - the father met her not so long ago before he passed away. Maybe 2-3 years. She already had a child (Tommy) but he's not MC's outlaw brother. We can see she lives in a nice house, which may as well be an extortion from MC's father or other men. Also Theresa is not that far from Dizzy as a short form. She seems the right age for such affair. But here is the secret sauce of the ridiculous part - her religious craze ISN'T her true self. It's her conditions. Her whole life, she's been a gold-digger who chased men for their money, but now she has to abandon sex and male relationships all-together in order to take the money. So she decided the best way is to turn to religion and that's what her messed her up that bad. We can see she urges to fuck the MC, but seems to stop herself unwillingly. This is my main theory.
Jolina - there's something fishy about that girl. Idk what it is, but something doesn't feel right about her. She pretends to help the MC, but wouldn't be surprised if she turns up to be the ex-"gold-digger" (putting on quotes here cos till this point we can see the so called gold-digger Elaine actually cared about that ass). She was left some money to buy the place and turn it into a profitable business and she chose a cafe. She mentioned that after her grandmother died she met someone. Imo that's exactly the MC's father, but my guess is she didn't have any clue he was married and had children and later when she found out what an asshole he is, she dumped him. But we know he said he cared about her. So that's his way of trying to turn her life around. Probably she has a clause to prohibit her of telling anyone about her part until the final reading of the will. This one is a long shot at best.
About Lily - she's too young to have had a significant enough relationship with the father to the point where he would put her in his will. Also this kind of stuff wouldn't have gone unnoticed by Katie and Debbie. Same goes for Debbie.