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.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or my firm or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice. IRS Circular 230 Disclosure: In order to comply with requirements imposed by the Internal Revenue Service, we inform you that any U.S. tax advice contained in this communication (including any attachments) is not intended to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing, or recommending to another party any transaction or matter addressed herein.