If he was her father, then the odds are that her mother would already know.
Perhaps the DNA kit was to prove it to him and/or to Hannah to stop any diddling that was going on -- and somebody flew off the handle at the results.
Could be..because a DNA kit is not something normally carried around in a car..the moment I heard that was found in her car my ears went up..they also found 2 used condoms in the garage(At least I think it was the garage) perhaps the mother knew her daughter and DiMaggio were having sex and was going to confront them both about it..the day of the fire was the 15th anniversary of when DiMaggio’s father kidnapped the daughter of his ex girlfriend and held her hostage til his suicide which was on the day of DiMaggio’s death