I think you must fully concentrate on giving strong financial and family reasons why the Mother and Father in law will return. Its a really difficult one because there may be no compelling ties, that's what makes it really easy for the UKBA to decline visit visas. As I mentioned before, the loved one was granted a visit visa for the UK (ten years back for a wedding) then refused two a couple of years later when I arrived on the scene.
My wifes friend was successful with her mothers visit visa. Family visit visa, I recall. She stayed for 6 months.
I read that family visa requires that you already have to be a permanent resident to avail of family visa