This illustrates a problem. His sister and he may have had a good relationship since childhood, and they are brothers and sisters, but her sister should not be jealous. After all, the person they like is going to spend a lifetime with him, and the two children should be well matched. As for you, you should also learn to blend in with your sister. The two brothers and sisters share the same blood and grew up together. You should learn to understand this friendship and tolerate some small willfulness of your sister. She may just be unaccustomed to her brother's sudden kindness to the second girl except her, but as long as you are considerate and sensible and let your sister see your kindness, I believe that if you are a sensible sister, you will bless you both and you have a good relationship with your sister. Isn't your boyfriend happier? Come on!