i dunno but since like forever i wondered abt Gunjan' Under confidence & bit of nerdy ways if i may say so but seeing bui i think i have my answers
i think after their mom passed away since bui has bought up GunUr she is also responsible for Gunjan being so introverted
as nupur may have been a bit older & hence she was more mature so as to speak she was not intimated by bui but may be Gunjan was & hence she became so meek as bui was so overpowering & may be looked after her more than Nupur (thus sparing her ) as she was younger