My girlfriend's brother sent me a short message. How should I reply?

Brother, thank you! I understand your intention. If I had a sister, I would do the same as you. In fact, in private, I am a person who is not good at words. I know that you have devoted your feelings to XX (your girlfriend's name) for so many years. Now, I will take over your baton, love her, take care of her, and give her the greatest happiness in her future life journey! Please believe me, believe in a sincere and persistent heart!