Teasing and playfulness have little to nothing to do with appropriateness - quite the opposite a lot of the time (as any engaged man "harassed" one too many times by his partner's female friends could tell you, believe me).
In an adult game? Certainly. Day to day? Not so much. Teasing does not equal flirting and the confusion many times leads to embarassment or worse.
Any particular reason you can think of for this suspicion? The one part where we got to see the family members thoughts on the poisoning seem to imply he was as shocked, if not more so, as the rest of the family.