How to be a good younger brother for the only elder sister?

We have posed the question to the "all-knowing experts" of nowadays – ChatGPT, Gemini, Llama – and their common advice is to be supportive and helpful, respect her space and show appreciation, and don't forget to have fun together.

Felix was the first sibling his elder sister Fanny got to know. They met when she was 3.5 years old. She was the eldest among them four, he was the second eldest, and it’s probably they two who had the strongest connection between a sister and a brother in their family.

The Mendelssohns' were building their relationship about 200 years ago and managed to do it well. So, what could Felix, the younger brother, tell us from his experience?