From what you state, it sounds like your brother's life estate is not a concurrent interest with the remaindermen so there isn't anything to partition. He could sell his life estate to anyone, including the remaindermen, but unless the purhaser has a remainder interest they want to consolidate, there probably isn't much value to the life estate. You need to sit down which an attorney who can review the documents before giving advise particularto your situation.
On what basis is the partition being sought? Was this through a trust? If the sisters are doing it, they may be challenging the trust or the Will.
The life estate holder has the right to occupy the property for life.
What are you doing and why?
You really need to sit down with an attorney.
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It depends on the language of the governing document (will, trust, etc.). It may be authorized but it may be prohibited also. Hard to tell without examination of those documents.