Oh, sweet little Annie. We really hope she isn't the "her," but like we said, just weighing our options here. After Déjà's act of anger at Kate's wedding, many people are wondering if Beth and Randall are going to focus all of their energy on Tess and Déjà. Tess, to make sure she's OK after the scary car ride that landed Kevin in jail, and Déjà given her difficult foster care upbringing. This would absolutely tie into Kevin's conversation with his family in rehab about how they were constantly making sure Kate and Randall were OK, but just assumed he was fine when he was struggling himself.
In all of the flash-forwards, we never see or hear anything about Annie. It's possible she's away at college during this conversation between Tess and Randall, but it's also possible that she's "her." Many comments on Reddit are connecting the genetic predisposition to addiction from William. With addiction in her family, is it possible they are nervous to go visit Annie in some form of rehab or mental health facility?
Reddit user grumpypanda91 even reminded us that when Annie was bonding with William she was fixated on the idea of "a vice," telling Randall that William's "vice is cocaine, and mine is asthma." Although she's still very young, this could be the show's way of exploring her character.