Mind your own business to an extent, if u r REALLY worried that people may stir the situation and make it bad for her, then you could suggest to her that holding hands with dewar might look a bit odd. But like u said, her hubby is ok with it.
Personally I wouldn't hold my dewar's hand, even if he is a bit younger.
^ Yeah, I thought the hand-holding was a bit odd too, Halwa.
I say pull your cousin to the side and talk to her. Tell her that although you know that she loves her husband and has no romantic feelings for her devar.............society has a bad habit of wagging their tongues over the smallest of things. This way..........you won't sound like you're accusing her of having a crush on her devar. Instead you'll just be giving her friendly general advice about how conservative desi society thinks. Know what I mean?
Another thing to consider is.........was your cousin close to her devar BEFORE marriage also? I mean did they hold hands BEFORE marriage too? If they did..........then it wouldn't seem unusual to others because that's the type of relationship they always shared. BUT.............if the hand holding suddenly started AFTER she got married....................then it has the potential to raise some eyebrows.