In fact, in this case, since you say you are willing to give up, it is wise to buy it directly for your sister-in-law and then complain to your sister-in-law about your in-laws. They will naturally know, be embarrassed and change their behavior.

And tearing B is the most irrational and hurtful thing. Some of their reactions are normal. Can only say ignorance, can't say that you did something wrong. In-laws have problems, so don't talk about them. Let their sons talk about them themselves.

It's not your fault. It's about teaching you to be smart. Find a solution to the problem instead of simply venting, otherwise it will only bring new problems.

PS: You can disapprove of their practices, but you have no right to criticize their ignorance. They have the power of ignorance.