Well everyone knows that Sedna's Yetis are a bit of a waste.  She has too many other good abilities and they are too expensive and fragile to make a difference.  Therefore I present a completely different alternative:

Calm:
Enemy minions lose the will to fight and potentially join forces with Sedna

Level 1:  Enemy minion/creep attack speed reduced by 10%; 10% chance /sec that minion/creep changes ownership to Sedna. Sedna may control up to 2 extra minions.  Minions return to their owner if calm is deactivated
Level 4:  Enemy minion/creep attack speed reduced by 15%; 20% chance /sec that minion/creep changes ownership to Sedna. Sedna may control up to 4 extra minions.  Minions return to their owner if calm is deactivated
Level 7:  Enemy minion/creep attack speed reduced by 20%; 30% chance /sec that minion/creep changes ownership to Sedna. Sedna may control up to 6 extra minions.  Minions return to their owner if calm is deactivated
Level 10:  Enemy minion/creep attack speed reduced by 25%; 40% chance /sec that minion/creep changes ownership to Sedna. Sedna may control up to 8 extra minions.  Minions return to their owner if calm is deactivated

Range:  15 (Same as Healing Wind)
Cost  :  Calm can be activated and deactivated like Ooze.  Cost is 10/15/20/25 mana per second.

Ressurection:
Sedna's minions return to life to defend her once more.
Level 15:  Minions under Sedna's control that die within the effect of Calm have a 50% chance of returning to life at 50% health.

Rationale:  This is a fairly expensive skill, as it is a constant drain on mana when active and restoring mana is harder than restoring health.  But it presents a completely different build for Sedna.  She has the ability to convert portions of a creep wave, which, while not that powerful to start, could be tremendously powerful near the end game (imagine if she calms your giants!).  She also has the ability to steal other demigod's minions.  And, in general, it helps her resist getting swarmed under by enemy minions.  Her principal weakness is her lack of AoE and her slow attack speed.  This gives her an AoE effect (conversion) and the ability to slow down the damage of attacking creep waves.  However, because of the high cost, Sedna would have to sacrafice something for this ability.  So you wouldn't have to worry about a healing, pouncing, silencing Sedna.  A viable Calm build would also need at least two mana items to stay on the field.  The costs and effect percentages are just guesses, and playtesting would be needed to find the proper balance.