Njal explained the current situation to his brother with impeccable details, Gissur was in disbelief because he could not believe the Ásbirningar clan dared to touch on their own.
Gissur knew he could not war on two fronts but this only made them warier of the Ásbirningar clan.
If they touched one of their own, it was clear that they were now officially joining the war and Gissur had no problem with that.
He knew a few of his men would be more than enough to dismantle the Ásbirningar's clan.
He had let them be because he did not see them worthy to be conquest and despite their supposed transgressions, he still did not worry about them.
Njal was going to join the main forces with part of their army, they had a little over 1,500 men and women under their command and there was no way he would give his brother the majority of his army.