What is the name of Applejack's younger sister?

Applejack’s younger sister is named Apple Bloom.

Apple Bloom is an earth-pony filly who lives with Applejack and Big McIntosh at Sweet Apple Acres in Ponyville. She is a member of the Apple family and is known for her energetic personality, inventive plans, and determination to discover her special talent. Her older siblings help run the family’s apple farm, while Apple Bloom often takes the lead in the group’s adventures.

She forms the Cutie Mark Crusaders with Sweetie Belle and Scootaloo after the three fillies bond over not yet having cutie marks. Their early adventures center on trying different activities in hopes of discovering their individual talents. In the season-five episode “Crusaders of the Lost Mark,” the trio finally earns cutie marks representing their shared ability to help others find their purposes.

Apple Bloom is sometimes confused with other apple-themed names in the series, including Apple Fritter and Apple Betty. Those are separate characters, not Applejack’s sister. Apple Bloom is also Big McIntosh’s younger sister and Granny Smith’s granddaughter.