In Sick Boy they have to adapt to a drastic change of living so becoming stick-figures might play off quite similar for the most part. Poor Mr. stick tho, no hands and no more pillowy lips XD
Evan Moriarty (big brother)
1) would probably check to see what happened to his manhood 1st XD
2) he's an opportunist so he'd go on to test his stick figure limits
3) His usual routine is to hawk a loogie onto his hands to manifest weapons:
He'll first test to see if his none-existent mouth allows such. If not he'll find new methods
4) he would bury any concerns with comic-relief and finding opponents to test his capabilities 
Owen Moriarty (little brother)
1) he would be scared and confused at first, seeing Evan's unfazed personality may help him calm down
2) he would wonder if he can still infect people in this world and if his superpowers are still intact and maybe if his camcorder works too (other life-forms turn into monsters when infected by these brothers)
3) Owen would be dragged into Evan's wild adventures throughout the stick figure land, despite his wishes 
