I'll meet with my bfs family at Chuseok

Good luck!! 😅 If you don't know Korean well make sure that you let his family members know as well beforehand so that they won't misunderstand you. Other than that, your body gesture will say a lot. When saying, thank you/sorry/hello/bye make sure that you bow for them slightly to show respect. When receiving/giving something make sure that you do it with two hands always. Whether it's an item or a food. Doesn't matter. :)

Oh there's a character limit 😅😅 I'll continue here. When it comes to speaking in Korean, make sure that you speak with honorifics to his family. Such as whatever you say, finish the sentence with 입니다 (eubnida) when asking questions add 요(yo) at the end of your questions. These things will help you come across as respectful as possible. But in general, if you have a good attitude and respect for them, they will see it regardless. You don't need to worry. It will be fun :)