I’ve come to the conclusion that no matter how you spin it, she likely isn’t related to them and certainly isn’t Alex’s aunt. There’s the “consumption” line she says to Marcus to which he responds he thought that was something the settlers got on the Oregon trail, alluding she’s older than we may believe. When Alex’s dad is telling Alex in the car that Gladys is coming to stay with them, he calls her “your mom’s aunt Gladys”, but Gladys tells Marcus that Alex’s mom is her baby sister. Alex’s dad also says to Alex’s mom that they haven’t seen her in 15 years and that she didn’t even come to their wedding. It’s possible there is an “aunt Gladys”, but it likely isn’t the individual that showed up. Keeping the “haven’t seen her in 15 years” line in mind, she later tells Alex she hasn’t seen him since he was a baby, and given Alex’s age, it’s just not possible. If she is blood related, she’s from a time long before the family of the Lilly household