Why it could be her:
- Sansa has plenty of reasons to hate Joffrey, the biggest being his role in the murder of her family, including her father, her mother, and her brother Robb.
- Right after Joffrey dies, Ser Dontos Hollard comes to Sansa's side, urging her to come with him if she hopes to survive, implying that she has some sort of hand in the murder.
- After the wine goblet falls, it's Sansa who hands it to Tyrion to give to the king, so she had access and could have added something to the cup.
Why it couldn't be her:
- Simply put, Sansa is too obvious a suspect.
- We see that Petyr Baelish had orchestrated Sansa's escape and that she didn't know about it — and she also asserts her innocence.