* Is it possible to take personal accident policy with my brother in laws family also
No, You can take a policy individually or in a group. But the group should not be formed specially for taking the PA policy.

* Are there companies who offer personal accident policies which are not dependent on the IT returns you file. As I understand the PA policies of PSU insurance companies depends on the IT returns you file. You can PA policies up to 100 times your monthly income.
Income proof is mandatory for Sum Insured more than 2 Lakhs in Public Companies.

* Are any companies offering “floater ” personal accident policies.
No.

* I have SECURED FUTURE plan of Tata AIG. In case of any unfortunate accident, can the company from whom I take PA policy refuse compensation ( as I already have secured future plan of TATA AIG). I have also taken JANTA PERSONAL ACCIDENT POLICY from oriental insurance. Do these policies clash ?
For some parts 'Yes' for some parts 'No'.
Yes - For the claim of Income Loss.
No - For claim of 'Death' or loss of any other body parts etc.

* Are there any companies which offer PA policies for more than 1 year term.
Yes, 'Secured Future plan' of Tata AIG is a long term plan.