yeah your sand sifter's gonna starve.... just kind of the way it is, really....
i probably wouldn't read too much into the fact that the sand sifter died.... but i would be VERY curious (as you are) as to why that brittle died.
how long have you had them? and of course since it's an invert i have to ask about your acclimation method.
you probably mentioned all this in other threads.... but i haven't seen it.... can you mention some info about your tank, params, all that good stuff :P
like also mentioned before, things such as copper kill inverts, and not always instantly. was this a brand new tank when you got it, or was it used? i like to gather lots of info :P