Is the speaker considered sinful for causing his stepmother's surgical operation to not take place, given that the operation was postponed at his request without his father's knowledge?

There is no sin upon you for delaying your stepmother's surgery and prioritizing your father's surgery, because you intended good by expediting your father's treatment, and no harm resulted from postponing the surgery for your stepmother. The postponement was also with her consent. The principle in such cases is to prioritize what is more important; thus, cancer treatment is more critical than gallbladder treatment. The reference for this is to specialists. You can now assist in arranging her surgery, but if you do not, there is no sin upon you.