The Elder Brother (Quiet Isle)'s wiki page starts off like this:

The Elder Brother is the leader of a septry on the Quiet Isle. He is reputed to have powers as a healer. This elder brother's given name is unknown.

So I thought why not, lets look into this a bit.

The Elder Brother's Identity and Potential Powers

Note: Sandor Clegane is no longer the Hound.

Identity

Background

Large square head, shrewd eyes, heavy jaw:

The Elder Brother was not what Brienne had expected. He could hardly be called elder, for a start; whereas the brothers weeding in the garden had had the stooped shoulders and bent backs of old men, he stood straight and tall, and moved with the vigor of a man in the prime of his years. Nor did he have the gentle, kindly face she expected of a healer. His head was large and square, his eyes shrewd, his nose veined and red. Though he wore a tonsure, his scalp was as stubbly as his heavy jaw. -AFFC, Brienne VI

44 years of age:

"You are young, child. I have counted four-and-forty name days . . . which makes me more than twice your age, I think. Would it surprise you to learn that I was once a knight?"

"No. You look more like a knight than you do a holy man." It was written in his chest and shoulders, and across that thick square jaw. "Why would you give up knighthood?"

The thirdborn son in a family of knights:

"I never chose it. My father was a knight, and his before him. So were my brothers, every one. I was trained for battle since the day they deemed me old enough to hold a wooden sword. I saw my share of them, and did not disgrace myself. I had women too, and there I did disgrace myself, for some I took by force. There was a girl I wished to marry, the younger daughter of a petty lord, but I was my father's thirdborn son and had neither land nor wealth to offer her . . . only a sword, a horse, a shield. All in all, I was a sad man. When I was not fighting, I was drunk. My life was writ in red, in blood and wine."

Loyalists during Robert's Rebellion:

"When I died in the Battle of the Trident. I fought for Prince Rhaegar, though he never knew my name. I could not tell you why, save that the lord I served served a lord who served a lord who had decided to support the dragon rather than the stag. -AFFC, Brienne VI

Potential Option****s

Using the information above I thought it would be best to look at landed knights that might share some similar features.

But as I was going through the landed knights page on the wiki, I came across one that smacked me square in the face.

House Brune of Brownhollow

House Brune of Brownhollow is "an old knightly family from Crackclaw Point", which was known to be "dragon men" and to have fought with Rhaegar on the trident:

One night he had told her that he was kin to the Brunes of Brownhollow, an old knightly family from Crackclaw Point. -AFFC, Alayne II

and:

we bow only to our own lords, and the king. The true king, not Robert and his ilk." He spat. "There was Crabbs and Brunes and Boggses with Prince Rhaegar on the Trident, and in the Kingsguard too. A Hardy, a Cave, a Pyne, and three Crabbs, Clement and Rupert and Clarence the Short. Six foot tall, he was, but short compared to the real Ser Clarence. We're all good dragon men, up Crackclaw way." -AFFC, Brienne IV

If interested: Legends and Myths of Crackclaw Point

We also get Ser Lothor Brune's appearance (which is somewhat similar to the Elder Brother's):

Alayne wondered what Mya made of Ser Lothor. With his squashed nose, square jaw, and nap of woolly grey hair, Brune could not be called comely, but he was not ugly either. It is a common face but an honest one. Though he had risen to knighthood, Ser Lothor's birth had been very low. One night he had told her that he was kin to the Brunes of Brownhollow, an old knightly family from Crackclaw Point. "I went to them when my father died," he confessed, "but they shat on me, and said I was no blood of theirs." He would not speak of what happened after that, except to say that he had learned all he knew of arms the hard way. -AFFC, Alayne II

Feel free to share if you have a better idea, but this one matched most of the known information we know about the Elder Brother's past.

Healing Powers

Throughout the series we see magic attributed to different religions, such as R'hllor and the Drowned God. Very little is attributed to the Seven (some potential visions, aid in trial by Seven, etc.). But it should be noted that we get this little bit about the Elder Brother:

"Yes, brother." Brienne unpinned her hair and shook it out. "Do you have no women here?"

"Not at present," said Narbert. "Those women who do visit come to us sick or hurt, or heavy with child. The Seven have blessed our Elder Brother with healing hands. He has restored many a man to health that even the maesters could not cure, and many a woman too."

"I am not sick or hurt or heavy with child."

and Brienne makes two other "references" to it:

Nor did he have the gentle, kindly face she expected of a healer.

and:

He looks more like a man made to break bones than to heal one

And while I couldn't find anything else on his healing powers it should be noted that it leads right into a discussion about Sandor Clegane (the former Hound).

He has restored many a man to health that even the maesters could not cure, and many a woman too."

"I am not sick or hurt or heavy with child."

"Lady Brienne is a warrior maid," confided Septon Meribald, "hunting for the Hound."

"Aye?" Narbert seemed taken aback. "To what end?"

If interested: Arya Stark: The Key to Jaime/Brienne & Lady Stoneheart

Healing Sandor

This seems like something any normal maester would do:

You would have pitied him as well, if you had seen him at the end. I came upon him by the Trident, drawn by his cries of pain. He begged me for the gift of mercy, but I am sworn not to kill again. Instead, I bathed his fevered brow with river water, and gave him wine to drink and a poultice for his wound, but my efforts were too little and too late.

and we see the Sandor/Gravedigger very much alive (but lame):

higher still they passed a lichyard where a brother bigger than Brienne was struggling to dig a grave. From the way he moved, it was plain to see that he was lame.

So obviously this is a way that could make Cleganebowl more realistic.

TLDR: The Elder Brother was potentially a landed knight from House Brune of Brownhollow, and seemingly has some "healing powers" that could be used in the future.