It would make sense that each of you owned the 26.666 in equal shares of 8.8886 acres each. That being said, each owner is separate of other owners even if you own in the same tract or piece of land. Because your brother sold his acres it will not affect your ownership. You still have the same ability to lease or sell your minerals. There are no repercussions for you and your sister because your brother sold his share, things go about as normal and you have complete control over your 8.8886 acres.
Do you have a copy of the Deed that gave you the acreage? Unless the deed has some weird language I should be correct and it should be your sole and seperate property from your brother and sister.
I'll be more than happy to answer any other questions.
Cameron is correct. Your and your Sister's interest are stand alone interests. Your brother's selling has nothing to do with yours' ownership.
And I don't think you need to pay anyone to run your title out. That can be very expensive (possibly thousands upon thousands of dollars) and since you appear to already know what you own, you don't need to spend the money.
If you are interested in the history of your interests anyway, you can add a provision to your next lease requiring that the company that leases it provide you with any Title Run Sheets or Title Opinions they prepare or obtain covering your lands. That is free.