Verin was not looking to poison Cadsuane out of rage or anything. She was cold-bloodedly preparing to eliminate Cadsuane to prevent her from imposing an agenda on Rand, which has been Verin's MO from day one. She was glad when Cadsuane answered that she had no intention of doing anything to Rand beyond correcting his personality flaws, which means that Cadsuane was highly unlikely to pick up any ill-intentions.

In addition, that is an absurd amount of cleverness that only works in hindsight. If she detected a sister she did not really trust feeling angry or hostile towards her, or detected the presence of poison as someone was serving her tea, opening up and confiding further is an absolutely senseless response tactic. Rather, it would be a situation where she should clam up and never again let herself or Rand be alone with Verin again, and have her loyalists keep a close watch on her.