Were you a remainderman? Was your sister? Did you only purchase the one-half (or part) she inherited?
Property with a reserved life estate typically receives a step up in basis to the fair market value as of the date of death of the decedent. The share you inherited would receive that stepped up basis while the basis of the part you purchased would be whatever you paid for it.
Since the payments are spread over more than one year, you would enter the part you purchased as an installment sale. In TurboTax, you can enter "installment sales" in the search box and then click the "Jump to" link in order to enter the transaction.
