What must a thief do after repenting, especially given that the one who was stolen from is reciting Surah Ya-Sin against the thief?

All praise is due to Allah for guiding you to repentance. Part of complete repentance is returning stolen goods to their owner. Therefore, you must return the jewelry to your grandmother, even if indirectly, as long as she has not absolved you of it. As for Surat Ya-Sin, there is no authentic evidence of its effect on a thief. And if Allah accepts your repentance and is pleased with you, then nothing will harm you, by Allah's permission.