Alright I breezed through the episode. So here is what I found.
1) Only the main guy and his kids do any signing, we never see it from the other Betazoids.
2) The two siblings only do it with their father. There is one scene where the 3 are together and they do it. The other is when they are with some federation people.
So the idea that the kids use sign language with their dad, makes sense. The idea that they use it when federation people are around to be polite, makes sense. The only questionable scene is when they do it just them as a family (rather than speak telepathically)..... is it because the girl has that inhibitor and its actually easier for her to sign than talk in her head? Unknown. But I agree that this is a very minor scene.
There is one technical goof in the episode I caught on this run through. When the dad comes into the whale tank to scold his daughter, he is speaking, but if you watch the scene he is just walking towards them with his hands at his sides, so a small goof, again pretty minor.
So in conclusion, I think the sign language is self contained enough. We can look at it as purely a reflection of a family that has a deaf person, rather than a sudden change in betazoid culture on the whole. So....all is well I'd say.
