Ian knows that he had a twin brother who died when they were both tiny; he's asked questions about what happened, and we've answered them.  I make sure that Ian knows that he, Ian, is loved just for being Ian, not more or less because we miss having his twin with us.

It's hard to know exactly how Ian has processed it, though.  He has high-functioning autism, and has difficulty with language function.  Although he is verbal, he has a tough time putting together complex verbal expressions.  But I know that he thinks about it, because he's told others that he had a twin brother who died when they were both babies.