GMantis

Not really. What was explained to her was that Sweetrobin could be given two more doses as long as he wasn't given any more in six months, so as far as Sansa knows he's not currently in danger of suffering long term effects.

Sansa does have such a tendency, but the fact remains that the last time she gave him sweetsleep was before she learned of Littlefinger's plot about her marrying Harrold Hardying. At that time, it would make no sense for Littlefinger to want Sweetrobin dead, since his whole power rested upon being his Lord Protector, so Sansa would have no conceivable reason to believe he was being poisoned. She might be closing her eyes about him being poisoned after that but this isn't the same as poisoning him herself.

First of all, the last time Sansa gave Robert sweetsleep, it was with a maester's permission (A Feast for Crow, chapter 41: Alayne II): Second (and following from the first) she can hardly be part of a plot to poison Robert when she learned of the plot only after administering the sweetsleep - she only learned of Littlefinger's plan after they went down from the Eyrie, where the above conversation happened. Third, Sansa definitely doesn't want Robert to die (The Winds of Winter, sample chapter: Alayne): So it's clear that Sansa isn't participating in any plot to kill Sweetrobin. At most you can accuse her of pretending not to notice others poisoning Sweetrobin, but there is hardly any evidence to reach such a conclusion.