Taleroth: In this case, the journal is incorrect. There is no evidence to support that she did the poisoning, only what she did afterwards.
It may be accidentally left in from a previous version of the game where that was indeed the plot, but it's not how things are in the release version.
Tristanian: Or the reference in the cutscene might have been left there from a previous version, take your pick.
No evidence you say ? You find a book of poisons and antidotes in her house. Gerald himself states that such books describe both the poison and particular antidote in great detail. Philippa knew that the poison that would otherwise kill any ordinary man, would not have such a severe effect on Saskia due to her dragon origins. She knew exactly how to treat it. It was the perfect opportunity to charm her and manipulate her. She had both the motive and the tools.
On the other hand there are only circumstantial evidence against Stennis. Suspect : Thorak clearly proves that it was the priest that performed the actual poisoning deed, so to speak but it is never clearly revealed whether Stennis ordered the poisoning.
Either way there is no hard evidence against either party (eg a testimony of a witness) but in my eyes at least, Philippa had a lot more to gain and relatively little to lose when compared to Stennis. He is a lot of things but I find it hard to believe that even in his arrogance, he wouldn't realize that such an action would diminish his chances of truly being recognized as a ruler, considering that a poisoning would inevitably arouse suspicion and lead to the event as we experience them.
You are way off base. that book and poison was in refernece to the rose of rememberance in the CURE. As others have said she didn't even know Saskia was a dragon till after she was poisoned.
The biggest proof against Stennis is that he allowed the priest access. he knew the plan and whether or not he thought it up he supported it and allowed it to continue. He also refused to give any blood to help cure her.
And by all indications they meant to KILL Saskia. So who wants her dead? Philippia surely didn't. She wanted to control her not kill her. But the nobles and Stennis did want her dead.