You should consult an experienced Oil and Gas Attorney for anything of this nature. The potential future value of any mineral rights should be closely reviewed before making any decisions, especially any decisions about selling them.
There is a woman in New York City that accepted my advice and did not sell her mineral interests in what you might think of as a rinky-dink 50 acre tract down in the Eagle Ford Shale play south of San Antonio, Texas.
Instead of accepting a quick deal, for an amount of money that she probably would have gone through by now, she is now included in 3 producing wells with a total of 8 wells possible (and probable), and that is just from the Eagle Ford Shale formation (there are others known to be productive in the area).
I conservatively estimate that her small interests in the 3 currently producing Eagle Ford wells will be making her $50,000 a month in royalty income by the end of this year, with the additional wells still to be developed.
That's A MONTH, Babette.
The people making offers to buy you out know something that you do not - the amount of money they have offered you reflects that.
If you will post your legal description(s), I'll see what I can find out about what is happening in your area.
In the meantime, hang tight and don't sign or agree to anything. I have a feeling you are in for some very wonderful surprises in the coming months - perhaps a lifetime of them.
It is a shame your sister acted so hastily. If you have other family members - cousins, etc - with interests in the same or other area lands, you may want to forwarn them about taking the quick money. At least until I have a chance to take a look.
If they are still tempted, or need the money for some reason, then ask them for the first right of refusal to buy them at the best offer they received. Keep as many of the interests in the family, so to speak.
Hope this helps -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas