The most obvious word that comes to mind is typical:
Having the distinctive qualities of a particular type of person or thing; characteristic of a particular person or thing (ODO)
In your example, the most normal way to phrase this would be to use typical as an adjective modifying the thing that is typical of someone, rather than as an adverb or sentential adverb to comment on the way in which the person is doing it. In other words:
Now she was smiling with her typical sincerity.
Some common synonyms for this usage would be usual and characteristic.
If you are specifically looking for a phrase that you can insert into your sentence at that exact place, the most idiomatic phrasing I can think of is as was her wont.