What is the ruling on refraining from giving truthful testimony for a relative in an ongoing dispute between two of them, in order to avoid suspicion or ill-will from the other party, whether the testimony is given individually (as an attorney/representative) or in the presence of both parties (for evidence), taking into account that refusing to testify may anger the one seeking it?

Your testimony has two scenarios:

1. If there are other witnesses besides you, then you are not obligated to offer your testimony unless it becomes incumbent upon you; because in this case, it is a communal obligation (fard kifaya).

2. If your testimony is the sole means to establish the truth and you are certain of it, then it is an individual obligation (fard ayn), and you must offer it. It is not permissible to abandon it merely due to embarrassment.