There are many ways to reply to someone wishing you a happy Raksha Bandhan, but here are some examples of replies to a sister:

• "Dearest sister, thank you so much for your wishes. Happy Raksha Bandhan to you too!"
• "Thank you for being such an amazing sister. Your love and support mean the world to me. Happy Rakhi!"
• "Waiting for this day all year long, when you lovingly tie the Rakhi on my wrist and pray for my well-being, fills my heart with joy. Thank you for your wishes, dear sister."

These are just a few examples, and you can personalize your reply based on your relationship with your sister and what she means to you.