Hi snarling_wolfy. I don't think you can use 'decent' as the antonym for 'indecent' in this context. I think 'acceptable' or 'appropriate' would be better. I think any other preposition used in place of 'in' might make it ambiguous. If behaviour is acceptable 'for' someone, it means it is acceptable 'to' them i.e. it is acceptable in their opinion.

examples: The clown's behaviour before the crowd was hilarious. Behaviour in a library should by quiet and respectful. The behaviour of some prisoners is frightening. In your sentence, as it is framed, in is the only one that could be used, but if this is what you want to say, maybe 'Hermione's behaviour, reading in bed, was unorthodox but predictable.' might be better.

Generally, I'd say 'from' is the most common and natural. "I won't stand for that sort of behavior from you!" The use of 'in' in this sentence however, is a very pleasant and rich alternative. Still, I think that in the case of behavior, which is such an external thing, 'from' is generally better. You might consider 'in' for something more essential or internal, like 'intelligence'. She shows behavior, but has intelligence. "I see a darkness in you." "I see a behavior in you"? Less natural. Here it might be 'from' rather than 'in'. Having said all that, I don't think anyone would misunderstand or pause at "indecent for anybody" and I think it works just fine, correct or not.

From is OK too.