Final answer:
The sentence can be shortened to "Selvi resented Keerthy for consistently outperforming her academically, causing her father to boast of Keerthy's achievements," which maintains the original meaning in fewer words.
Step-by-step explanation:
To make the line "Selvi was annoyed by Keerthy because when they were teens, Keerthy would get 100 marks in all subjects whereas Selvi wouldn't get 100 marks, and so her (Selvi's) dad would be talking proudly about Keerthy" shorter, focus on eliminating unnecessary details and restructuring the sentence for brevity while retaining the original meaning. A possible revision could be: "Selvi resented Keerthy for consistently outperforming her academically, causing her father to boast of Keerthy's achievements." This concise version reduces the original sentence by more than half and conveys the essential information in a clear and direct manner.