You ask what are your best and worst case scenarios concerning sexual abuse allegations by your step-daughter. Clearly being found not guilty will be your best outcome and being found guilty will be your worst. If you are found guilty you may end up with little or not jail time, probation or many years in prison, if depends on too many factors and future outcomes that are just not knowable with certainty. You have a serious criminal case which requires an experienced, defense attorney at the helm to guide you through this. Under no circumstances talk to the police or anyone else about this without a lawyer present for you. They are not kidding when they say - "what you say can and will be used against you in a court of law."
Kansas Sentencing Guidelines are significantly more certain than federal guidelines but there are still many variable which can effect sentence length. Your prior history, the exact factual allegations, the judge, the prosecutor, the victim's attitude, and the victim's ages all can impact the length of a sentence. The double rule, the double double rule and departures are also important concepts.
In a case like this you need the help of an experienced criminal defense attorney who has handled and actually tried many of these cases. There is a world of difference between these cases and other types of criminal cases. The difference between having represented people charged with cex crimes with children and actually trying those cases is also huge. Do not talk about the allegations with anyone other than an attorney. Anything you say to anyone else could be used against you. Anything you say to anyone else could be misinterpreted and used against you. Contact an attorney immediately.
Legal disclaimer: Legal disclaimer: Patrick M. Lewis, (913) 558-3961, firstname.lastname@example.org. This answer is intended to provide general information about the justice system. It does not provide legal advice nor does it create an attorney-client relationship. It does not provide the basis for making decisions about a course of action. Legal advice requires more communication and information than is possible in this format. Many important considerations and factors need to be investigated and discussed before an attorney could give legal advice about this issue. Before making any decisions about a course of action readers are strongly encouraged to contact a lawyer and secure an attorney-client relationship. Readers must also understand that this format does not provide for confidential communication.
The worst case senario is prison followed by parole and lifetime sex offender registration. The best-case scenario is acquittal or dismissal. This is a dangerous allegation and one that you need to have the assistance of counsel with significant experience. If you do not already have an attorney, you need to interview several attorneys quickly and pick the one you are most comfortable with.