The main plot of the manga and the two anime series revolve around Ed scouring the earth for a way to restore his younger brother Al's body to normal. On the other hand, considering Al is inhabiting an animated suit of armor much larger than Ed, there's a quite unusual dynamic going on there. Especially considering how Ed reacts to anyone calling him short.
In the manga and Brotherhood, Ed sacrificed his arm to keep his brother's soul in existence. And later, his use of alchemy altogether. If that's not Big Brother Instinct, what is?
The Elrics both have this dynamic towards each other. Edward was more than once willing to die for Al (until he realized that if he died no one else would be able to put Al back to normal) — and Al did not approve of Edward's plan to let himself killed by Scar so latter one would leave Al alone.
At the end of Fullmetal Alchemist (2003), Alphonse sacrifices his recently Philosopher's Stoned self to revive his brother. Edward answers this with sacrificing himself to bring Al back. In the movie, they spend much time, stress and tears to find each other again after this ordeal, and although they succeed, circumstances are such that Edward decides to leave his brother for good and seal the Gate from his side, in order to protect Al and Amestris from Dietlinde Eckhart — and Al replies with following him to the far side to be with him. That's FMA for you...
The first anime and one of the novels also feature the Tringham brothers Russell and Fletcher. Russell is very protective of his brother, though it takes him a while to realize that otherwise he's been neglecting him. He gets better.
The first anime just straight up ups the quantity of siblings with protective instincts. As mentioned above, the Tringham brothers with very similar dynamics to the Elrics appear in the first anime, the Slicer brothers' connection to each other is focused on, a Scar who's haunted by his own brother's choices more-or-less adopts two younger Ishbalan brothers (coincidentally also similar to Ed and Al, with the older resentful of his mother for allegedly abandoning them), Winry's ship tease with Ed is greatly reduced as they grow older and she stresses about not being able to reach them as a sister (with Hughes reassuring her), and due to the changed background of Homunculi, Gluttony not only more-or-less reacts heartbroken when Lust dies near the end of the series, but Envy is the rare clear case of an aversion within the anime, due to their shocking connection to certain unexpected people they absolutely despise, and aiding in corrupting Wrath.
