Can my dad petition for my stepmother?

Depending upon several details, including your age when your father remarried, you may be able to file a petition for your step-mother. Alternatively, now that your father is a Permanent Resident, he may petition for his wife in the Family-Based Second Preference (F2A) visa category - but be aware that there now is a backlog of about two years for visas in this category. Your stepmother's sisters, who I presume are U.S. citizens, also could petition for her, but that would be in the Family-Based Fourth Preference (F4) visa category, and the backlog for visas in that category is very long - while it may appear shorter, in fact it likely to be 15 - 20 years or even longer, and so that rarely is a satisfactory strategy for attaining immigration benefits. It would be wise for your family to consult with an immigration attorney who, after learning all of the relevant details, could advise about eligibilities, options and strategies. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.

Your dad can petition for her but there is a line. He cannot petition for her sisters. Only your stepmom will be able to petition for them once she becomes a US citizen.