Is it obligatory for the mother and sister to wear the legal hijab when speaking to their son or brother via video call, especially if eavesdropping on the conversation is ruled out?

May Allah reward you for your diligence in safeguarding your family. There is no objection to your brother not enjoining them to wear hijab unless it is highly probable that the call will be spied upon and their 'awrah (private parts) exposed. What you are doing is better and more scrupulous, as it eliminates this possibility, unless the possibility of spying is impossible, in which case there would be no need for it.