Is it wrong for a sister to wears trousers in an environment where it is yet to be acceptable by some of the residents? For instance, in my country, some people believe trouser wearing by women is sinful. What do you think?
The idea of modesty is to not call attention to yourself due to what you are wearing. If you are working in a business where all the men wear button-down shirts and ties, then a Christian wears a similar outfit to blend in. This doesn't mean if everyone wearing too little clothing that a Christian is to join them. At the moment, I am focusing on societal expectations.
"For though I am free from all men, I have made myself a slave to all, so that I may win more. To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law; to those who are without law, as without law, though not being without the law of God but under the law of Christ, so that I might win those who are without law. To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means save some. I do all things for the sake of the gospel, so that I may become a fellow partaker of it" (I Corinthians 9:19-23).
If a woman is in an environment where all the women are wearing dresses, then she should be wearing a dress as well to blend in. If she is in an environment, such as on a college campus, and many of the women are wearing slacks, then she can choose what she wants to wear. See Is it wrong for a Christian woman to wear pants?
Yes, there will always be people who dislike all changes, even if those changes are not necessarily wrong. When you are dealing with such people, you try to adapt to their expectations. However, it doesn't mean you have to meet their expectations even when they are not there.