Kath can see clearly that Fin and Daniel aren't the only ones who are paying for Daniel's decision: Tom and Kylie have effectively lost their mother and as far as Kath knows, nobody else is stepping up to do much (as evidenced, in her eyes, by Dad and Brendan's unwillingness to force Mum out of bed). Bringing Kath into all of this makes it clear that in order for the family to move forward, it's going to need to be a group effort and everyone will need to accept that they're accountable to more people than they initially thought.