Everything makes sense now that the in-laws are appearing in the story. Although we were not shown any of their actions, plans, or precautions early on, we can assume that they were ready to step in when it is necessary and be the "heroes" and "caring family"(when her life is threatened or possibility of being maimed, etc), driving a wedge between her and her parents. Note that from all the subtle hints we were shown, this family is quite ruthless and ambitious, so who cares if she gets hit by her boyfriend or gets put in danger, they wont step in selflessly just for that...
What they did not factor was Nobuto (that his name?) being an absolute retard who might have killed her in a fit (since they might be too late to intervene, also shows their selfishness) and Tetsuo being a SuperChaDad, most likely rendering their plans moot.