Is it obligatory to inform the siblings about a brother's financial transgression in the inheritance, knowing that this will cause significant disputes and quarrels?

If you are certain that your siblings have a right to the inheritance held by your eldest brother, and no one else knows this besides you, then you must inform them. This is because concealing testimony leads to the loss of rights, unless there is significant harm to you or to them in bearing witness, and not merely the occurrence of dispute or contention.