Respect and humility are enough, and you don't need to be particularly affected to do something nice, because your boyfriend's sister will have many long-term opportunities in the future. You can't be good every time, and people's thinking will become habitual. You have been kind to her twice and done one thing, but it may not last long. When one day you neglect, she will think that you have changed.

Therefore, it is enough to treat her as a friend, respect her opinions and take proper care of her.

How to get a person's approval may not need a gift, because this is not a workplace or an interest society, and what is needed may be just an attitude.