Is it right for Employer to ask Candidate to fill up the Job Application Form before the interview?
As the Form requires a lot of privacy like Family members's name, their employers etc

Based on my experience, it's necessary to provide only after interview (if they keen to offer), right ?

Depends on company process.
I have filled in forms before interview, after interview or upon being offered the job.

You can leave part of the form blank if you think it intrudes your privacy.

For financial services company, yes. They need to see politically exposed individuals as well as connected parties. I also believe the same for audit companies as well.

just fill up the sections u feel is relevant. they will come back to u if u did not fill in some inportant details

I always filled in forms before interview,

Yes, it is a hassle.