I can;t find the part of the rulebook which actually governs how Stratems ar eused, but as I iunderstand them, they interrupt normal play. In this case, I think SfD would be considered a part of the normal flow of the game where Ynnari are concerned, so a Strategem could interrupt it.
In that case, if killed something, and then declared Soulburst to either make a new Charge more or Fight another unit, the Strategem WOULD interrupt it and take place first.
Also, Soulburst is only activated AFTER the entire Fight sequence has finished. So AFTER the consoloidation move.
So the correct sequence of events, as I think I understand them based on how describe, was:
1 - Yvraine charges.
2 - Yvraine fights, and kills the Broodlord.
3 - At THIS point, the Strategem is used, allowing the Broodlord to fight Yvraine.
4 - In your case, the Broodlord killed Yvraine.
Had Yvraine survived, and as you wanted to use her, she would have:
4 - Yvraine complete hers Fight phase by Consolidation into the nearby genestealers.
5 - NOW, you trigger Soulburst on Yvraine to have another Fight phase, during which she fights the Genestealers.
"Models that charged this turn can only target enemy units that the charged in the previous phase."
Soulburst allows you to fight as if its the Fight phase. I DON'T think this overrides the limit above, so you couldn't fight the Genestealers, because you charged someone else this turn, and not the Genestealers.