Well, I don't know what his will says, if he had one, but what likely happened is this: When you purchase real property with a spouse, it is usually deeded as "joint tenants with right of survivorship." Meaning that, should one of them die, the other automatically assumes full ownership of the property, regardless of what their will says. It doesn't get probated, there's nothing else that needs to be done; it's just theirs. When your mother died, likely, your stepfather became the sole owner of the house. Now that he's died, the house will go to his heirs/beneficiaries under his will.
The best thing to do, however, is to take all of the documents you have to a local attorney to review, so you know for sure. But the likelihood is, yes, his children are correct in saying you and your siblings do not have any rights to the property.
