Can I petition for my fiance and my soon to be step-daughter?

Do none. Just apply for a fiance visa. Best is to hire an experienced immigration attorney

Very unfortunate timing. After you get married, you can file for your spouse as he is an "immediate relative" as the spouse of a US citizen. The problem is that his daughter is already 18, and to create the qualifying step-parent/step-child relationship; the marriage must be completed before she turns 18. Since you are not yet married and she is already 18, you can never petition for her, as she will never qualify as a child of a US citizen for immigration purposes. Adoption will not work either, as the adoption must be completed prior to the child turning 16 for immigration purposes. After her father is an LPR (about 6 months), he can start the petition process for her, but since he will only be a permanent resident, his daughter will be a FB-2A, minor child of an LPR, with a priority date backlog of approximately 3 years for most countries. She can then apply for her immigrant visa. If she is going to college outside the US, her permanent resident immigrant visa should be ready for her by the time she graduates (4 year degree). Consult with an experienced immigration attorney.

You may petition for your fiance, but not for your soon to be step-daughter since she, having reached the age of 18, would not be considered your "stepchild" under the INA.