Marcus was a step behind her the entire way, even when she had dashed into her sister's room.  He had stumbled once on the stairs but quickly bolted after her when get his balance. It didn't take much to figure out what was going on even if Rin's screaming was deafening.  
And as much as he wanted to join her in beating the hateful man to a pulp he ran at her instead and hooked his arms around her waist from behind to drag her bodily off the other.