Still under installment would mean that there is an existing charge/assignment in favour of the bank. In other words, the bank is technically the beneficial owner of the property. You can either approach the bank and request to do a transfer subject to charge (if there's individual title). The bank can allow you do so subject to certain conditions, or request that a refinance plus transfer is done. Most likely, they would ask you to refinance. In both circumstances, you'd require a lawyer to do it.

If refinance, you and your sis is basically taking a new loan to settle your parents' loan and then a transfer of ownership is done. Legal fees are according to the loan sum taken and can be calculated at the first page of this thread using the calculator at the link provided.

thanks alot. ill discuss with my sis and parent. =)