Doh sleep on Sansa... remember the look of death she give him while de imps and dem was acting out the battle scenes. Plus she's the one who hand Tyrion de goblet from under the table. Thing is he drink from it and was fine... until de pumpkin pie, lol
While they are both annoying, Sansa eh have belly like Khaleesi to pull off something this vile.
Without giving the plot away, think about who (family) has the most to gain with Joffery out of the picture.
Also, the killer would have been within arms reach of Joffery during the wedding to have access to his goblet.
You are on the right path with your observation that Sansa gave Tyrion the goblet. But the the goblet was passed along like a relay baton, so look at it from that standpoint.