I agree with Abhilasha and interest..... only my take is slightly different.... Remember epi 19th feb, part 4, when dev saves radhika from drowning (one of my 50 top RaDev moments !!!) And after rubbing her feet in an effort to revive her... and finally when she becomes conscious.... he says ~ main apni had janta hoon aur mere sanskar mujhe ye had paar karne ki ijaazat nahin dete (English ? PW, anu..)... not to mention the fact that at that time he didn't even know she was vish's sister...
So, why didn't Dev put his hand on Rad's shoulder ?.... my answer would be, being somebody else's husband and a RP, he is not only not supposed to touch her, but also coz he's bound by his sanskars (upbringing) which dont allow him to touch her.... and that's why he's trying very hard to control his impulses.... But as far as holding her hand in open daylight to save her during her kathin tapasya (the word he used is pran ) is certainly not considered "touching" her....
Another corollary I can think of is when he brings her back inside the house (epi 6/16, last part I think) after keshav opens the double door) from the clutches of the producer, and is holding both her shoulders (aaw...) in front of entire family..... that was not touching either..... and daadi and rest of the purohit khandaan saw the plight she was in....
My 2c !
~P~