Hi Allison this can happen when Ancestor's are also 2nd, 3rd Cousins ect.- related to one another when they get Married. This Happened in my Mother's Lines in New Brunswick, Canada, it was common in that Province. My Maternal Grand Parents where 2nd Cousins, but that was due to there common ancestor getting Married Twice also making my Mom's Paternal Grand Parents related 3rd Cousins when they got married. it was an ancestor of my Great Grand Parents that married Twice making them all related .So it may be Possible that your Mom's Half Brother may also be a Cousin to her.
i know this all seems funny but it is what it is Circles in a Family Tree