i am really sorry to hear of your daughter's plight and can empathise v.well with her.
the best thing is that her inlaws support her
i also agree with the views of 498A victim,esp. the 3rd point.
is the girl aware that ur son inlaw is married?if yes,i will say that she's still v.selfish and stubborn who's not leaving him.
i will add one point to 498A victim's suggestions that public humiliation of the gal is v.imp....threaten her once.if she doesnt budge an inch, humiliate her publicly after deciding the proper way to do so.
u can also sit back with ur samdhis(daughter's sasural wale) and decide a solution on how to get rid of this girl.
there's a possibility that after humiliation the gal may walk out but ur son inlaw may go after another woman,esp if he has the habit of eyeing other girs.in such a situation leniency wont pay any further...u may file DV Act against him,showing all proofs...because laato ke bhoot baato se nahi maante...this formula has to be applied both on your damaad and on that girl...such people need to be dealt with an iron hand and deserve no mercy which has already been shown,esp. to your son inlaw.
all the best.please let us know the results.
(PS-i hope indira jaisingh reads this story and realises wot sort of women she's wanting to be given highly respectable names)