Remember that no one can take advantage of you unless you LET them!
Unless her parents were rendered incapable of logical reasoning due to some slowly delibatating disease, they willingly made the decision to allow the couple to live with them (for love of their daughter, I suppose). I would probably do the same for my daughter (and son-in-law) if they fell on hard times.
Consider the dilemma though should they take action...they grow tired of the bum and hand him his walking papers, but wait! Their daughter is married to him, so she would have to go as well...so you're throwing out the baby with the bathwater!
It's a delicate situation. But there is little that can be done unless the daughter concedes to leaving the house to strike out for themselves as a couple (like every other adult does).
I have had my share of houseguests (on my wife's side), and it's amazing how accommodating they expect you do be, without feeling any obligation to pull their own weight. I couldn't do much either, because I would be locking horns with my wife! Fortunately, they eventually left, and I made a new decree:
No one without a permanent residence of their own is allowed to stay at our house for more than three days!