Been a while since I drew Sarah - for those with short memories (or new to my arts), Sarah is the future first adopted child of Ellie and Rachel. In the future canon (as it currently stands anyway!), Rachel proposes to Ellie on Christmas Day 2028, they marry in 2030, and adopt Sarah in 2031 when she's a toddler.
(There's another adopted daughter as well, but - with apologies to who I got her from - I've had to rename her after advice from a friend, and need to update her introduction and character sheet accordingly!)