You have not specified under what provision and act your sister in-law filed case. An FIR under 498A or a petition under domestic violence act, or a complaint under dowry prohibition act..
If your sister in law has lodged an FIR, during the bail and trial times you can defend to disprove their claims.
If they have made a cash transaction , without making any documentary or witness evidence, or bank entry of transfer, it is difficult to prove.
If it is a dowry case 498A IPC, during the trial your lawyer can establish her claims as false.
Before that , you should try quashing the FIR if it is a criminal case under 498A.
If it is a case claiming the under DV act or in a civil suit, She has to prove the amount paid , if the contractor being a relative is an unreliable witness.