The expression "trés petit" or "tp", used for labeling clothes, comes from the French words trés, which means "very", and petit, which means "small". Thus, such expression is indeed the same as using the "xs" in English, standing for extra small. In countries where both languages are used, it is common to see both labels on the tag.
The answer b states that "from then on" meaning her behavior has changed toward her brother and the passage states how she has been putting a little extra thought into how she interacts with her brother.
