In this context, to be mortified means <em>to be embarrassed, to be humiliated.</em>
Explanation:
We can analyze the meaning by studying the word of origin: <em>mortificare</em>. If, like it says in the given statement, mortificare in Latin means <em>to cause death</em>, we can infer how Janis felt, after she realized her brother had read her diary. She felt so embarrassed and humiliated that she probably felt like dying, <u>in a figurative way</u>. <em>She was so mortified, she felt she was dying.</em>