This is termed Dharmasankatam (conflict between the moral code and the actual situation), an ethical quandary.
Notes:
Bhisma, Dhrona, and others were ensnared with dharmasankatam. They were aware of the unjust happenings that were beyond their control.
Those assembled then were under King Diridharastra’s rule and they still put forth their opinions at opportune times.
Those days they did not interfere with the king in his court as it was considered disrespectful.
When Dharmaputra questioned Bhisma on his silence, Bhisma replied that he had faith in Dharmaputra as he was always virtuous, that he shall not indulge in wrongful deeds and there was always a righteous reason behind his every move.
But the crucial question is why did Dharmaputra relent? Everybody had faith in Dharmaputra that he would not err. When the tussle among family members increased, he obeyed his uncle without questioning because he held his uncle Dhiridarastran in high esteem.
Bhisma was restrained due to various dharmic reasons like patriotism, being grateful, etc. it is difficult for us to understand such matters. Only by perusing Mahabharata thoroughly, we can arrive at an understanding.