Writers Game: Whodunnit?

Finals [LILLIAN JONES]

"Marie! You want anything for breakfast?" Lillian shouted from the dining room. I peeked at her from behind the door, careful so that she wouldn't see me.
"Uh, yeah can you make me a frappaccino please!" Marie answered from the parlor, leaning over a book and studying it very closely.
As Lillian turned, I darted back from the doorway, hiding in the kitchen closet. I heard a creak from the door opening, and slowed my breathing, making it almost silent. She couldn't know I was in here.
The small closet was cramped with cooking supplies; whisks and mixers, bowls, and whatnot. I pushed myself back as far as I could from the door so that no one would be able to see me from underneath. I had to go incognito for this. No one will see it coming.
I heard Lillian moving around in the kitchen, pulling out coffee beans and the frappaccino maker, along with some mugs and biscuits. I waited, quietly, feeling the container in my pocket. The right dose of this will get her for sure. Poor girl.
It was Oleander, a deadly flowering shrub. All parts are poisonous, and for this particular case, I have the nectar, hard to detect in an ingested substance. It was perfect.
Suddenly, I saw feet move across the light path underneath the doorway, and I froze, holding my breath and drawing all my limbs as close my body as I could. The feet stopped in front, and stood there, doing something on the counter across from the door.
I waited like that for a while, anticipating when it was safe to move. When she would go away.
"Lillian!" Marie shouted from the parlor. She was obviously getting impatient. Just like her.
"Yes?" Lillian answered, pulling away from her spot and drawing nearer to the exit of the room.
"Can you come here for a second? I need to ask you some thing about this book."
"Of course, be there in a second!" She came back, adjusted something, and then I saw her footsteps leave and heard her exit the kitchen.
Now was the time for someone to die. I very quietly, but quickly, left the closet and moved towards the mugs of coffee. Pulling out the container of Oleander, I put a lethal dose in one of the mugs; not enough to be detected, but enough to kill her.
Footsteps were coming closer to the door, so I capped the small bottle and darted out of the other doorway, ducking behind a bookcase.
"Alright, coffee's ready!" Lillian picked up both mugs and walked into the parlor. I had a perfect view of my soon-to-be killing.
As they sat down, Lillian handed Marie one of the mugs, but I couldn't tell which one it was; they were identical, both of them frappaccinos as well. I just hoped my plan worked.
They both took sips, studying the newspaper on the table before them, Marie's book beside it. Lillian pulled out a pen and circled a few things, and Marie set down her mug.
I was patiently waiting when Marie started sweating and vomiting up her drink and food.
"Marie! What's wrong? Oh my god, I'm calling the police. Hang on!" Lillian reached over to grab her phone off of the table, and Marie continued to vomit and sweat, as well as start to look a little woozy; just what Oleander is supposed to do.
She fell onto the floor, unconscious, and Lillian dropped the phone to help. "Oh, please Marie, wake up. Wake up!" Lillian was crying, checking her wrists for her pulse and listening to her heart to make sure she was still alive.
Eventually, she stopped trying, she knew that Marie was dead; my plan had worked perfectly.