Totally awkward moment just a hour ago. PLEASE HELP!?

i have 4 of my friends from cheerleading over right now and things arent going ok. were 13 and all way taller than my older brother who is 16 and only 4'8", my shortest friend is 5'5" im 5'7" the other 3 are like 5'6"-5'8". we were in my room when they saw my brother walk p in just his dumb wall-e tightiewhities. they all laughed and couldnt believe he still wore little kids briefs. but i told them since hes so little he still loves wearing them. so without me knowing cause i was in my bathroom, they snuck into my brothers room and went thru his undies drawer and found a new pack of spiderman briefs my mom must have just got him. they took them and ran back into my room. when i came out they had already opend the pack and were putting them on laughing so i joined in. it was so funny cause my brother has a little tush so they were kinda small on us since were all way way taller than him. so we were just running around in my room in the spiderman underwear trying to act as if we were spiderman by jumping off my bed when my brother knocked on my door to come in. we all stopped and just stood there. before i could tell them to hurry up and change back into our undies and shorts, one of my friends laughing as she did it opend my door. my brother walked in and saw all of us in his underwear. thats where i thought he was going to kill me but instead he laughed so one of my friends came up from behind him and put him on her shoulders letting him think he could fly. he seemed to like it and when she put him down heres where everything went horrible. he was still in just his walle underwear while we were just standing around him when we all saw the front part of his underwear kinda move up. at first i didnt notice cause i would never look down at my older brothers thing but then it was completely noticed when his underwear like stretched out forward ewww. he was so embarred and just ran out of my room. my friends were really grossed out at first but when we realized it was kinda our fault mean even though he's older than us by like 3 years he really is like a younger boy physically so now were trying to think of him as a 6 year old to clear our mind but cant. please help i know this is incredibly long but what can we do to act like this never happend? should we go into his room and apologize? what should we wear? mean b4 we were just wearing cheershorts and tanks but we're afraid that might even get him nervous. Please what should we do?