While it is important to emphasize that Iris is certainly not responsible for Laura’s suicide, it is also clear that she let her sister down in a major way. Crucially, this was a repeat of an earlier time when Iris let Laura down, something that Laura has explicitly brought up to her before: when the girls were children, Iris did not act on Laura’s statement that Mr. Erskine was molesting her (and in fact didn’t even believe her). Here, Iris appears to have done a similar thing by ignoring Laura’s heavy-handed hints about Richard.