Can I force a step mother to probate my dads will?

Yes, as a potential heir ou can file a claim with the court and force production of the will. Whether a probate is necessary depends on several factors and you can't force a probate where one is not required.

You can certainly start an estate proceeding yourself in New York as the surviving children. You can require the production of the Will and have it probated in the Surrogate's Court. However, probating the Will does not necessarily mean that you are entitled to any assets. The Will only governs those assets that do not have named beneficiaries and do not have suriving joint owners. If your father and step-mother owned their assets jointly, or if your father listed her as the only beneficiary, the Will does not provide for other ownership. Their are expections to the rule and the particular circumstances surrounding beneficiary designations and joint ownership can change the result.