How will Portia react when she knows that Brutus killed Caesar?
Interesting question! Well, there are only a few brief mentions of her in the play. At the end of Act 2, Scene 1, Brutus agrees to tell her his "secret", and reveal to her the plan to kill Caesar. He does so (offstage) and we next see her in Act 2, Scene 4, almost maddened by the knowledge.
She even seems to be hearing bizarre noises:
Hark, boy, what noise is that?
I hear none, madam.
Prithee, listen well.
I heard a bustling rumor like a fray,
And the wind brings it from the Capitol.
This is clearly someone going mad through strain and stress: and it doesn't take a genius to connect her madness with her knowledge of the conspiracy.
And, after that scene, we never see her again. The play makes no suggestion how she would have reacted to the news that the conspiracy was successful: though, as she is worried mainly in this scene that Brutus will be found out before he has killed Caesar, she presumably would be happy that her husband had been successful.
For the record, the end of "distract" (mad) Portia is related in the tent scene: she died because she was
Impatient of my absence,
And grief that young Octavius with Mark Antony
Have made themselves so strong: for with her death
That tidings came, with this she fell distract,
And, her attendants absent, swallow'd fire.