How old is the tank, and what are your parameters? Do you have a picture?
Seastars are some of the most delicate animals we try to keep.
Brittlestars are some of the hardiest types, relatively. It is unlikely, BTW, that a sand sifter and a brittlestar had issues...more likely something else happened. Standard sand sifter stars are not predatory on something like that (some are).
A Linckia might be suitable for your tank but it depends on a lot of things, and IMO, 100lbs is on the low end for LR. Reef safe seastars can not be spot fed, and rely on mature LR. Many will die of starvation in 12 to 18 months.