Becca is intended to be his biological child. There are lots of hints in the story. Frank was sleeping with Shannon, Becca's mother, 9+ months before Becca was born. Shannon was shipped off to her aunt suddenly 9 months before Becca was born. Shannon didn't meet Trevor (Lillith's father) until after she was pregnant. Trevor cannot be Becca's father, and the only guy we know to be sleeping with Shannon around the time of conception is Frank. Shannon was most likely shipped off to hide her pregnancy. She didn't tell Frank the truth because it would make trouble for her. Frank hasn't put it together, but Becca has done the math and has her suspicions in her side story.2 are step, Abby is from a previous relationship (her mom was was already pregnant when MC met her). Dunno who else is supposedly related, I gave up and rage quit after I realized that. And I dunno why whether confirming it or not even matters to Patreon, lately they've been going by apparent intent rather than actually included or not, so even alluding to stepcest like this one obviously does will get it put under review and probably banned, probably for similar reasons why you can't even legally make age play porn (every actress 18+ with paperwork to prove it but looking and acting like they are 12 for example) without the feds coming down on you like a ton of bricks if they are made aware of it, which has happened to a few OF starlets for their custom clips they sold after they were snitched on.
When Patreon suspended his account, the dev said that he can now tell Becca's parentage. Though, he was reinstated back to Patreon so we will have to see if that gets revealed or not.