My boyfriend insists on taking his sister on a trip

Of course, let him go. Although your sister may be an adult in your eyes, in the eyes of your family, she will always be a child. And everyone has different experiences. Many girls are really always accompanied by their families. I have been somewhere before, so I am very timid and my family is not at ease. If you don't let him go, you feel very inconsiderate. What a great opportunity to show your generosity. How can you not let him go?

Not only should you let him go, but you should also say polite words such as "take care of your sister and be careful along the way", and act like a big sister in front of her sister to make her like you.