Susan would have attended because hi, it's her uncle! But she probably wouldn't have known too many people since they were all her aunt and uncle's friends and... not hers. She'd say hi and try to chat with the people she knew that weren't her aunties, without being too awkward (even if she felt like it).
She would have gotten Hannah to make him a basket of sweets, and gotten him a book on a topic he likes from Bookends.