I guess bcoz of their natures. Siya loves Riddhima in general, so if she is saying something about her, there might be some truth.
Whereas Ishani keeps abusing Riddhima. So he doesnt take her seriously always.
Vansh knows the natures of both his sisters
He knows how innocent siya is and how cunning Ishani could be at times and he knows that Ishani hates Riddhima also