Specifically, the final repeating phase. Everything up to that point is peachy keen. But once she splits and there are tornadoes separating us the S hits the F.

I've tried pretty much every variation I've heard, read, or seen videos of. The thing that ends up killing every group I've been in comes down to not being able to kill both sisters in time, or 'voking the Spiny and having it not stick to the voker, even after a Lob/Tomahawk, with or without DoT's on it (CoS is very guilty of this).

There are also several times when I'll voke the other sister as OT, and even though I've got aggro on the aggro list, she stays over there for an extra 15s or so, and the melee and I are just left sitting on our hands. This is after I get thrown every which way from those damn tornadoes of course.

What am I doing wrong?

Also, 40s Cd for provoke has killed me more times in that fight than I can remember. If I'm OTing and we kill my sister fast enough, we just end up sitting there for 8-12 seconds for my voke to cooldown, simply because I had to blow it on the Spiny as soon as it spawns (unless it's one of those rare instances where it starts on a non-MT target). That damn feather seems to generate a lot of threat on it's first target, such that it'll be dead before you take aggro from anything non-tank or healer.