Can I withdraw the I-130 petition for my step daughter which is now waiting for the date of interview?

More information is needed. If all of the documents 'were submitted' to the National Visa Center, including the I-864 and DS-230, then you will likely have to contact the consulate or embassy that will process the visa make sure that the petition is properly revoked. This can get complicated, since she is just waiting for the interview. The visa packet may still be approved and issued unless you can get acknowledgement that the visa is revoked by the consulate or embassy. You should consider hiring a competent attorney to try to confirm the revocation is completed. You will need to take care of this as soon as possible and need to properly document any and all action taken. This can get frustrating. If you are unsuccessful, you can be responsible for supporting your step daughter each year for the next ten years or until she becomes a naturalized U. S. Citizen, whichever comes first. If you or the attorney are unsuccessful, then you may be liable for over $100,000, perhaps $200,000 or more, for the duration of the ten year period. Good luck. The above is general information, not legal advice, and does not create an attorney client relationship. I recommend an appointment with a competent and experienced immigration attorney at a minimum, but some matters are best handled by an attorney.

Yes, you will receive a confirmation letter.