Dashrath has no morals. He is dhongi of the highest level. Its too gross to imagine that DIL and FIL would go to that length, but I have seen this depicted in movies a few times.
I have another thought, what if Dashrath had a relationship with this woman before she became his son's wife...and bringing her home as his son's wife was a way to keep her close. Gross...but could be possible. Otherwise why would a woman like her marry a guy who is physically challenged.