If two protagonists means two player characters then no thanks, changing PCs mid-game would ruin immersion for me.
I think our sibling will play a big role in the story though. They could become an antagonist, maybe they died during the Reaper war and serve as our motive to find a better home for humanity or perhaps they were killed in Andromeda and we seek to avenge their death. Or they could simply become a squadmate.
I think that would be awesome!
You play the game with both, brother and sister.
In the gameplay, you progresively torn them apart because politics, life, whatever. They don't necessarily fight against each other, they are just not so bonded as family as before.
At the end, in the final battle, you play as them both until a point where one of the two brothers, main protagonist are Virmired (Telltale games Game of throne style). You have to choose one of them. Maybe you prefer one over the other for their gender, or ther alignement (paragon/renegade), or because romance interest, or because you prefer a Voice Actor more than the other... At the end, one sibling dies heroically, the other is alive.
Next games, you have to recover from your sibling loss.