Eh, Wouldn't they know if there were giving birth to twins?

most of the time yes, but there are rare cases where the presence of twins was not discovered until the delivery room..even unltrasound can in rare cases miss one of them...one may hide behind the other.
I think the movie leaves it up the viewer to guess: they could well be unrelated girls that look very similar
they could be long lost twins that were separated at birth for various reasons: maybe Roger could only cope with one kid after his wifes death and his job was also demanding on top of it;the Callaways thought they were having one baby and twins were disovered upun delivery..when Kathy Callaway wasy dying the medical team worked on her and couldn't save her..to spare him te upset they didn't tell him about Amanda and she was put in the foster care system while Alyssa was given To Roger..and kept the details covered up
Maybe there were twins but it was assumed Amanda died along with her mother of the birth complications but lived and it was discovered later but Roger was not informed and she was put in the system
so many guesses

They're strangers! So Amanda's mom or Alyssa's mom didn't give birth to twins.
As much as the movie tells us, Amanda and Alyssa just happen to look "exactly" alike.
On the other hand, there have been cases of nurses, doctors, etc. stealing children. And that could have been the situation with Amanda and Alyssa. Amanda could have been stolen and put into an orphanage.
& Some women don't know that they're carrying twins until it's time to deliver. It happens.

"Do you even remember what you came here to find?"

Everyone really does have a doppelgänger. My best friend of 20 years ran into "me" at a hoagie shop.....even she couldn't tell it wasn't me. She took a picture of this girl and everyone I show it to is amazed. My point.....twins don't play a role here. Simply doppelgängers