They're really old projects both starting around 1990. Scilab was not free software (commercial distribution of modified Scilab was verboten) until they switched licenses to CeCiLL with Scilab v5 (2008?) which is a FSF recognized free software license and GPL compatible.
Unlike Octave, Scilab's goal is not compatibility with Matlab. Unmodified Matlab code will not run in Scilab. One of the goals of octave is to run Matlab-compatible code. Scilab has a comparatively limited syntax relative to Matlab and Octave. Octave's syntax is more expressive than Matlab's but currently lacks the new classdef OOP (Octave does have @class OOP) . The last time I checked Scilab didn't seem to have any OOP.
Unlike Octave, Scilab's goal is not compatibility with Matlab. Unmodified Matlab code will not run in Scilab. One of the goals of octave is to run Matlab-compatible code. Scilab has a comparatively limited syntax relative to Matlab and Octave. Octave's syntax is more expressive than Matlab's but currently lacks the new classdef OOP (Octave does have @class OOP) . The last time I checked Scilab didn't seem to have any OOP.