I know but hurts when brother/sister does it!
Why? Are you African American? I can understand your pain if that is the case. If you aren't, then from a moral stand point, you can disagree and correct people on their racist views.
Somalis are relatively new to America compared to other immigrants. So it is not out of place for them to identify themselves as Somali and not black because to them, they see the difference between having an identity based on national origin, and for the locals who were here for centuries and don't have attachment to their national origin but are identified by their skin color.
It is all about attachment to culture and identity. Somalis will always be Somalis. Down the road in 50 years, I think Somalis will identify less as Somalis but as African American.
You need to understand the dynamics of race and nationality.