Without knowing all the facts, if you have not had probate done in Oklahoma, then for marketable/clean title, that will need to be done. If your sister wants you to have her share, then a mineral deed signed and recorded by her will need to be done.
There may be other things as well.
If there was a trust, the question is whether your mother-in-law transferred the minerals into the trust. If so, it is a fairly straightforward matter of following the instructions in the trust regarding disbursement. If not, then a probate would be needed so that this can happen. Normally this is a fairly administrative action.
Notice:Informational only. No attorney-client relationship is formed by this post. I am an Oklahoma-licensed attorney, but this is not legal advice. Do not share confidential facts in this public space.