My boyfriend asked for leave to travel with me, and took his brother and sister-in-law with him, saying that he would go out for a day to liven up the atmosphere and not want to be too cold.

He may have a reason to take his brother and sister out. I guess he is not sure about you, so let his brother and sister go out to explore you, don't care too much. You should communicate as you please. Don't lose your dignity just because his brother and sister are here, but don't be too melodramatic, or they won't like you.