In the scene where the family is eating and Susannah is there for the first time... the men each raise their glass, except Tristan, who at first confusedly grabs his plate!
On the DVD version of the film, either this scene has been trimmed or you are a little bit mistaken. Actually there is no toast. When Susanne comes and sits down, everybody is somehow don't know the exact way to behave. Alfred offers bread. At that moment, Tristan holds a plate. When the camera view changes instantly, we saw that he is not holding the plate but his glass.
I believe this is an interesting insight into Tristan's character. The other men at the table offer her something out of politenss, but Tristan does nothing. This is showing that he is rebellious and not like the rest. I always enjoyed that scene. It's no slip-up.