Tatsumaki and Fubuki's past has not been easy. The revelation came some time ago when the psychic heroine reflected on her past and illustrated to the readers of One punch man the horrific experiments she was subjected to by a mysterious organization of this world whose origins are not yet well explained.
When this short part was presented in the manga drawn by Yusuke Murata, everyone focused on presenting Blast, but in reality, much more was told away from the spotlight. Another aspect of Tatsumaki's past was picked up in the final chapter of the manga, One Punch Man 182who, however, also raised her younger sister Fubuki.
At this point in the past Tatsumaki has already rebelled against the Tsukuyomi organization and he decided to go and save his younger sister who was locked in another cell where she was in the company of a dog. Tatsumaki's arrogance of just wanting to take Fubuki with him resulted in the poor little dog's death, which unleashed his younger sister's anger, helping her to exploit her psychic powers and thus also leading her down the painful path she is still walking today.
This bond between the two sisters is therefore riddled with bad feelings, albeit at the moment Fubuki doesn't seem to hate Tatsumakibut respect and reverence. However, Saitama's victory appears to have restored peace after convincing the Tornado of Terror to surrender to the choice of Fubuki and his men. But now both must work together against the mysterious Tsukuyomi organization.