This came up the other night.

Batman is adjacent to Livewire, but in hindering terrain. Half the group says that Livewire could use RCE against Batman. The other half said no because the line of fire crosses hindering terrain and Batman is also in it. They are adjacent though so he could make a regular hit, why not ranged when he is an adjacent flying character?

It depends, is Batman in hindering terrain? If he isn't, then Live Wire would be able to make a ranged attack on Batman because Batman is not using hindering terrain for Stealth and Live Wire is a flier.

Livewire could not use RCE against Batman...

The rules say that in order to make a ranged combat attack (and use RCE) you need to draw LOF to the character, which is regardless of whether they are next to you or not. If Batman is in hindering terrain, then the LOF will be crossing hindering terrain and therefore is blocked because of the Batman TA (Stealth).