The way I understand it is that MC's dad was either dating or married to Tiffany when he died. She's not his bio mom because she died when he was young. Regardless, since they're his step family they all see it as wrong even though technically they're not step family anymore since the dad died. But Tiffany took MC in because he had nowhere else to go. My guess is Jestur just didn't want to use the cliche of step mom/sister.