There is no requirement that the estate attorney be indemnified in order to terminate the estate. That’s nice for him if it happens, and he should probably withdraw if it doesn’t, but in theory you could terminate the estate by agreement without the agreement of a particular attorney. Who is serving as executor? What does he or she think?
If the sibling in question wants to insist that the estate assets pay for the suit against the attorney, as opposed to financing it himself, then he shouldn’t agree to the informal accounting and distribution, even if the attorney isn’t indemnified. In that case, the executor would have to go to probate court to do a formal accounting, and the objecting sibling would have to raise his objections there. That’s probably not so inefficient a way of handling things. If the probate judge decides that there is no valid claim against the estate attorney, that will probably be the end of the matter. If he decides that there is a valid claim that shouldn’t be waived, I imagine a fairly quick settlement should follow, unless the estate gets too greedy.