Is the A-pillar of the newly launched Sierra any different from that of the Nexon and Harrier?

Yes, the Tata Sierra’s A-pillar is completely different from the Nexon’s or the Harrier’s. The Sierra sits on a new platform with unique dimensions and hard points - the structural reference positions used for manufacturing and crash performance. 

The Tata Nexon, in contrast, is based on Tata’s older X1 platform, which traces its roots back to the Indica Vista of 2008. So, there’s no structural link between the Sierra and the Nexon.