Hi,
As per the facts provided by you, it appears that your sister borrowed money on security of same property which is owned by your father only. Based on the income and repayment capacity of your sister, the money appears to be borrowed on the property owned by father only.
In this situation, total property is worth now (apporx.) Rs. 2.70 Cr. out of which Rs. 2.30 Cr. is your father's investment and Rs. 0.40 Cr. is your sister's investment. Typically rental income of this property should be shared in this ratio only.
However, since the facts point that your sister doesn't have any ownership either in land or building, because mere borrowing money on the security of father doesn't entitle your sister any ownership rights over the property.
Hence, your sister might not be eligible for any standard deduction or interest deduction on this account, in the absence of any ownership on the property.
If your sister wants to claim any benefit on account of interest/ standard deduction, then the ownership must be shared.
Regards,