You wouldn't need to increase their HP too considerably if you reduced the mana cost of summoning them.  Right now the cost of summoning Yeti's is just ridiculous (erebus gets minions at no mana cost, oak at practically no cost).  

HORN OF YETI III IN NO WAY SHOULD COST MORE THAN SILENCE!!! :/

I mean even compare it to QoT minons mana cost.  The cost of summoning shamblers doesn't increase going from SSII to SSIII (fixed at 625 mana).

Whereas HoYII costs 750 and HoYIII costs 950!!! -____-

So you don't have to look it up, here's the current situation:  HoYI - 550, HoYII - 750, HoYIII - 950, HoYIV - 1150

My suggestion would be something like:

HoYI       350

HoYII      450

HoYIII     550

HoYIIII    750

Increasing Sedna's base speed to 6.3 would be a nice indirect buff to Yeti's, but would bring other balance issues so I don't think that's the right buff to give. (e.g. why is DA's base speed still 6.0?!? But that's another topic altogether )

What I think you want to do with Yeti's is to find some kind of way to make yeti's gel with Sedna's other skills e.g. giving a 7% buff to heal per summoned yeti  (meaning heal IV could potentially heal by by 1920) or buffing pounce in some similar way giving some kind of other incentive to have them around.

I'm not saying that's the right buff or anything, just something that makes Yeti's fit a bit more with Sedna so a viable build could be designed around them.

This seems to be a re-occurring complaint, Sedna and QoT need to pay for their minions, and they aren't cheap, making it hard to recast in the field while also using abilities. At least QoT's minions are out of combat with their range so they have some survivability. Yetis are in the thick of it, and when enemy demigod AoEs start flying, they are hit.

Of particular punishment to Sedna is her Level II and Level III skills. QoT pays the same price at level II and III, where as Sedna pays more for level III over level II (also more then QoT's level III). This is a huge cost, because you need to cast this skill twice to max yetis to 4. How often do you think it is feasible to blow 1500 mana in the field for the worst minion army in the game?

More than that.  To summon 4 yetis costs 1900 mana :/

So yeah, whatever other buff is discussed later, the first and obvious buff is reducing the mana cost which, to put it bluntly, is currently a joke