Relationship screen is organized by family, so if you do not add half-siblings to a family, you don't see them with that family.
The idea is that if people lived together, you add them to a family so it is clear with the eye that these people had a close relationship. You add them however to the family with a non-birth relation to (one or all) of the parents.
General questions should be asked on gramps-users list, if you believe you found a bug, http://bugs.gramps-project.org
is the website to use.