An oil and gas attorney or a title company or oil and gas "land man" may be able to run the title on this property and find out what happened to the leases. If it turns out that your brother has somehow done wrong in his administration of the estate, you may need a litigation attorney with some knowledge of probate.
I am licensed only in Texas. Offering information of a general nature in response to a question is not intended to be legal advice in your state.
First things first -- kick your nephew out of your house.
If your have an interest in the small estate left by your mother, your brother has an obligation to let you know what is going on. You can force that legally if needed.
As to the leases on the 3 lots, if your husband, your brother, and your sister-in-law don't know or won't tell you, then a title company can help you.
Assuming you have an interest in the estate, your relative who is administering the estate has a duty to keep you updated and informed. Based on your situation, it appears that this person has no desire to actually keep you updated. Because of this, I think you owe it to yourself to investigate this on your own. This is more practical advice rather than legal advice. As long as you have the legal description of the properties, you can contact a title company in the County where the property is located and they can create a "title report." The cost for these reports should be around $200.00 per legal description. This report should reveal what your relative has done with the property in terms of conveyance (selling) or leasing the property. Once you have this report, you can take this to a real estate attorney or a litigation attorney so they can explain what rights you may have. In short, if you do a little of the factual investigation on your own, you will save some some money with your attorney and probably have a very productive meeting with the attorney should you decide to go this route.
Fitts Zehl LLP
2700 Post Oak Blvd., Suite 1120
Houston, Texas 77056