Is there any way I can legally enforce her to release the remaining 1/6 share (sister's) on payment of Rs 2 lacs as was done for other shareholders
No legal option can be exercised on this. It can be solve by negotiating the amount to be paid to her as compensation. This can be achieved in amicable talks only.
I am needing to find a way out to acquire her 1/6 share through payment of reasonable amount and not exorbitant market rate.
Please suggest any solution for my predicament.
The only option is to pacify her and make her to agree to the amount proposed by with plus or minus.
Legally she is the joint owner in the property and can create problem. However if she is not agreeing to the proposed amount, you may file a partition suit agaisnt her, let she keep coming to India for litigation frequently and after getting fed-up with frequent court hearings and proceedings, she may opt for the offer made by you.