As per the facts of your case, you are the only legal heir to inherit this property. You are the nearest legal heir who is surviving today. Hence, you have the right over that property. In this situation you should file a declaration suit under Section 34 of the Specific Relief Act 1963. The court shall declare your right in that two flats.
Thus you can claim a property belonging to your unmarried stepbrother. After getting a declaration of the competent court in respect of that property, you can apply for mutation of the flat in your name. That decree shall have effect as right in rem hence, no one can challenge your right in that property.