Is steady breath good on a darkbreath f!corrin

If you are running steady breath, always lightning breath

F!Corrin specializes in Speed and Defense, so Steady Breath isn't too bad of a choice on her, allowing her to activate Ignis/Bonfire more frequently.

Lightning Breath on F!Corrin, in my opinion, is kind of a waste since her resistance stat is pretty low on its own, and her attack stat isn't all that great either.

Windsweep vs Watersweep - I dunno, really. I prefer F!Corrin to be more of an enemy phase unit, so both of those don't necessarily help all too much, but if you prefer to use her as a player phase unit, then it could be beneficial depending on the build.

SB +windsweep
And keep her Dark breath.

You player phase, lower atk and speed, then in enemy phase proc the special.

I recommend not using Steady Breath on a Dark Breath Corrin, depending on the stats. Personally I run a +Spd/-Hp Corrin with Dark Breath Attack refined. Lightning Breath isn't ideal for Corrin as it is weaker than Dark Breath and Corrin already has a moderately low attack., even after an attack refine. Personally I don't use Windsweep or Watersweep, but Vantage 3 with a Quick Riposte 3 Sacred Seal is perfect for a Dark Breath Corrin as long as she isn't -Def or -Res. If running Dark Breath, personally I'd run Distant Counter over Steady Breath, and Luna instead of Aether. However if running Aether, I can completely understand using Steady Breath. Just remember Corrin's weapon refined is basically Expiration but with a 1 Range. (That's why I use DC personally.)

As many noted, Lightning Breath isn't ideal.
Distant Counter may be a good call because even if she can't take magical hits very well, she will most likely kill archers and - more importantly - apply huge debuff against their entire team.
Windsweep and Watersweep are not great, but between the two, Windsweep is the better call by far - as a melee unit, she's not worried about counterattacks from casters anyway, and she does not perform well against other dragons.

However, if you plan to use her along other dragons, Windsweep allows her to play the crucial role of a Falchion counter rather well - as long as she engages the Falchion user first, she will do some damage and apply a _huge_ debuff. When the Falchion user attacks her next turn, she will certainly survive, and counter with Bonfire/Ignis/Aether. Windsweep gives her a chance to apply the debuffs for free - without the debuffs, she dies agains Falchion users unless she's running Triangle Adept, which is kind of a waste of an A slot.