When using cs_convert directly, the coordinates in input must be in meters, not in millimeters.
You should use something like this:
cs_convert(sMri, ‘mni’, ‘scs’, [-53 -87 44] ./ 1000) .* 1000
See the corresponding documentation here:
http://neuroimage.usc.edu/brainstorm/CoordinateSystems#Converting_between_coordinate_systems