Regina says she followed Daphne and Emmett for weeks after she let her ride on his motorcyle. Neither Daphne nor Emmett seem to know this. She's later revealed to have been following Bay since she was three.
Regina's line to her boyfriend about how him sleeping with his ex and her, "In my experience, history usually wins" makes much more sense after it's revealed that she knew about the switch.
Daphne tells Bay how, when they were eight, Emmett stood up for her and introduced her and her mom to sign language. She was three when she lost her hearing. Did it really take five years to figure this out? Most hearing people at least know the concept of signing...
That, and given that Daphne went deaf due to antibiotics given to her by doctors to combat the meningitis, you'd have thought the first thing the doctors would do after making sure she's recovered from meningitis, was to check her hearing then go through the options with Regina. Including (if not especially) sign language, reading lips and the like. If I were Regina I'd have sued those doctors for incompetence and negligence!
The Fridge Brilliance in that backstory is that a lot of doctors/audiologists are trained to see deafness pathologically in a "we must make this child normal" way. There is also the fact that a lot of hearing parents of deaf children have this vision of "perfect"/"normal" that they want their children to be, so when they're introduced to the oral method, they spring on it, clinging to any hope that their child can learn to speak and use their residual hearing in order to function like a "normal" hearing child. It makes perfect sense that Regina could have chosen oralism first before really being educated about ASL, Deaf culture, Bi-Bi, and the like.
Supported by the fact that while Daphne is perfectly capable of speaking and often does (as well as being given a cochlear implant in the Alternate Universe/What If? episode), Emmett and his mother rarely do.