[006:152] ....whenever ye speak, speak justly, even if a near relative is concerned; and fulfil the covenant of God: thus doth He command you, that ye may remember.
[004:135] O ye who believe! stand out firmly for justice, as witnesses to God, even as against yourselves, or your parents, or your kin, and whether it be (against) rich or poor: for God can best protect both. Follow not the lusts (of your hearts), lest ye swerve, and if ye distort (justice) or decline to do justice, verily God is well- acquainted with all that ye do.
A Muslim must always be upright and courageous, and always be ready to speak justice and truth irrespective of whom it is against, even if it's against his own self. So if you know there is truth to be told, then speak up.
Also regarding as witnesses to God, a witness has the moral obligation to speak up and to testify when he/she sees wrong. Covering up is not something an honest and true witness would do. So imagine how being a witness to Allah Almighty should be done. Even if it's a burden, one must do it as long as it brings no serious harm to you and to others. Must always act wisely.