We don't know much about Morshabaal. When it was announced by CipSoft, they did not give any insight about its story. If you ask NPCs that talk about Orshabaal, such as Avar Tar and Beatrice in Edron, none of them mention Morshabaal. Neither does other Edron leaders such as Daniel Steelsoul or Spectulus. So far, I haven't found any book mentioning Morshabaal as well.
It appears that all we know is what has been commented in the question: Morshabaal is Orshabaal's older brother. This information comes from two sources:
- Morshabaal yells "Revenge for my little brother!" and "I will restore our family honor!";
- When the teleport to his lair appears, the following message is broadcast: "Beware mortals! Orshabaal's brother, the infamous Morshbaal [sic] is out for revenge and will spread fear for his family once again!"
For now, this seems to be all the information available.