My boyfriend's sister-in-law asked me to buy her milk powder. I made it clear that my father bought it for her (my immigrant). She did not say a word of thanks.

Just because a boyfriend's sister-in-law has no culture doesn't mean that a boyfriend's family has no culture. Maybe her sister-in-law brought her bad habits, and the bad habits were brought from her home. I think you should know more about your boyfriend's family, his parents, brothers and younger brothers, and see if they are educated. Know clearly before you decide to break up.