Is it obligatory for me to create an Instagram account to denounce the reprehensible things I see in my sisters' accounts, or is it sufficient for me to use one of their accounts?

You are not permitted to disobey your father's command by visiting these websites, even if your intention is to forbid your sisters from engaging in reprehensible acts. Denounce to them whatever reprehensible acts you become aware of, using any available means that do not entail شرعية corruptions. It is permissible to boycott a perpetrator of evil if it is highly probable that doing so will achieve a benefit.