Family/Job question?

I work in a family run business. My father hired a temp before I started working at the business considers her a great employee. Out of fairness to her, when he hired me he made me her subordinate. I learned my responsibilities quickly and now find myself doing a lot of her work (i find her lazy - she merely tries to look busy). There is hardly enough work for the two of us to work an 8 hour day- and she gets payed while she commutes to work (I do not). We only work 32 hours a week and she somehow gets paid for 40 b/c she rides the bus. The business could lose her and productivity would not suffer. Because he hired us as temps we get no health coverage. I might add she is out of shape and is a huge medical liability risk in terms of costs of healthcare etc. I think she's a fat lazy slob-but now he wants to hire both of us full time (health coverage for both of us). I am payed well so it would be very difficult to find a job paying the same. Is there anything I can say?