I looked at the different synopsis' and all 2 trailers plus the tv spot and that's what I got from it. If Freya knew that Ravenna killed her baby why on earth would she resurrect her? Also in the TV spot there is a scene where Freya basically asks 'what did you do?' which is after the resurrection. So it looks to me that secret comes then for the first time.
I'm pretty sure it's explained that Freya fled the kingdom, went to a remote place, build her castle and basically raised an army (Huntsmen) to protect herself and maybe conquer the land. Then she banishes Sara and Eric for breaking her cardinal rule and is probably looking to kill them. Meanwhile those two team up with dwarfes and fight back. At which point Freya is trying to bring the mirror to her castle to resurrect her sister in order to double team Eric, Sara and so forth.
Once Ravenna is resurrected it isn't looking so rosy between the sisters (Ravenna basically says she will rule, her sister is weak etc. and Freya comes back with 'this is my kingdom.'). Once it comes out Ravenna killed her baby Freya will team up with Team!Huntsmen for both revenge and to not lose her kingdom.