Here's how we know Sansa isn't intentionally poisoning her cousin: we're in her head.
When she gives him the medicine, she thinks of how it will help him.
When she helps him down the mountain, she thinks of his safety.
If she wanted him dead she wouldn't be thinking that. Her internal thoughts have always reflected her true feelings, she had no issue wishing death on Joffrey. Even though she repeatedly wished and prayed for it, she was horrified at his actual death.
Seeing that happen, and then choosing to use poison again to kill one of her only remaining family members? Not happening.
Her thoughts of Sweetrobin are that he's annoying and immature. Not that she hates him and is looking forward to him being gone. She doesn't even seem to think about him being gone as a possibility. Instead, she thinks about how his wife will love his hair. She thinks about him ruling and wonders how he'll handle it.
She knows that Ned, Cat, and Robb are dead. She thinks that Bran and Rickon are dead. She thinks that Arya is dead. Her aunt is dead. Her grandfather is dead. Her uncle is hostage. Her last living family member is NOT dying at her hand.
Personally, I think he'll live. It ruins Baelish's plans and I want that to happen. He keeps underestimating his "pawns" and I want that to be another nail in his coffin.
