Let's take a look at few things..
1. Golu mentions her name as Suhana Bose to Dev.. At that point, Dev may have not understood obviously bcoz there may be hundreds of people with Bose surnames..
2. But the very next day, Sonakshi Bose arrives in the same school where Suhana Bose studies.. Coincidence? Dev didn't suspect it ??
3. Chalo, even if this all not possible, aaj toh bilkul clear hogaya.. Suhana spoke the same dialogue of her mother... These are enough points for Dev to alteast doubt that Suhana may be related to Sona...