Is testifying to the truth obligatory upon witnessing injustice, even if not requested, and if it displeases one's mother, would silence about it be considered injustice?

Bearing and rendering testimony is a communal obligation (fard kifayah). If requested from specific individuals, it becomes an individual obligation for them. Rendering testimony is not necessary unless it is requested, unless the rights-holder is unaware of it, in which case it is commendable to offer it proactively. However, if the rights-holder is aware, it is not appropriate to offer it proactively before being asked. If it is requested and becomes obligatory upon you, then it is not permissible to defy it due to the mother's dislike, because there is no obedience to a creature in disobedience to the Creator. The obligation to render testimony includes relatives and strangers, without concealment or favoritism.