Lucy Hale’s role as Katie Scherbatsky on How I Met Your Mother was surprisingly short-lived. Katie was Robin’s younger sister who always felt frustrated that Robin wouldn’t trust her to make her own decisions. She first appeared in the season 2 episode “First Time in New York” when she came to visit Robin in the city. After that, she wasn’t seen again until she attended Robin and Barney’s wedding in season 9’s “Vesuvius.” Considering she was a sibling of a main character, Katie was expected to have a larger role in the series, like Marshall’s brothers or Barney’s brother James. But Hale only appeared in those two episodes.

https://screenrant.com/how-i-met-your-mother-what-happened-robin-sister-katie/