Is there any relief in minimum marks required for the subjects for bihar board students?

No, eligibility criteria are same for all boards student for BITSAT.

For admission to any of the First Degree programmes of BITS except B.Pharm.: Candidates should have passed the 12th examination of 10+2 system from a recognized Central or State board or its equivalent with Physics, Chemistry and Mathematics (PCM) and adequate proficiency in English.

For admission to B.Pharm.: Candidates should have passed the 12th examination of 10+2 system from a recognized Central or State board or its equivalent with Physics, Chemistry and Biology (PCB) and adequate proficiency in English. However, candidates with PCM may also apply for Pharmacy program.

The candidate should have obtained a minimum of aggregate 75% marks in Physics, Chemistry and Mathematics subjects (if he/she has taken Mathematics in BITSAT) or a minimum of aggregate 75% marks in Physics, Chemistry and Biology subjects (if he/she has taken Biology in BITSAT) subjects in 12th examination, with at least 60% marks in each of the Physics, Chemistry, and Mathematics / Biology subjects.

For more detail kindly visit:-

http://www.bitsadmission.com/Eligibility_Criteria.aspx

All the very best and good luck.