UMMM they'd be limited-- but if he has an unhealthy attachment to her that particular echo would probably present to him as the way he remembered his sister, which means they could probably have any number of conversations as long as they're things they've more or less talked about before.

Echoes aren't logical and act on the layers that their owner has subconsciously added to them, basically. So she's like his sister, but just that little bit warped. Depending on how deep his refusal to acknowledge her death goes, the sister echo might reassure him that nope, nope, she's alive, right here kinda thing. Even if he occasionally sees her, you know, bleeding out, she's still alive. (I mean, if he has an unhealthy attachment and refuses to acknowledge her death and has recurring nightmares about her death, that's a conflicting bunch of things that could lead to nightmare fuel.)

....also, if he spends the entire time he's enchanted talking to her/choosing to believe she's real, she'll become far more fleshed out and real-ish, but that's another story entirely.