HELP, I walked in on my 8 & 13 yo old & my 8yo daughter's pants were down

I'm still in shock, shaking, and sick to my stomach on how to deal with this situation and I'm reaching out for advice. This happened just a couple of hours ago.A little background on my kids, my son is 13, has high functioning autism and his emotional maturity is maybe 10-11 years of age, however he hit puberty early and he looks more like he's 16. My daughter is 8 and has ADHD. They have a typical brother-sister relationship, meaning they usually can't stand each other and they're always bickering.Today I was working in the dining room and the kids were in my daughter's room, playing video games (on separate DS's) with the door open. Our rule is, if they're in a room together, door stays open. Mainly so if they start bickering about something, I can hear exactly what happened. I heard my daughter's bedroom door close. I figured they were just sneaking candy or something but I got up to check in on them almost immediately. I opened the door and my daughter was sitting on her heels with her pants & underwear down. The time between the door closing and me going to check on them was less than 1 minute. It took EVERYTHING I had to remain calm. I asked her why her pants were down and she wouldn't answer me. I asked her again and she said her brother pulled her pants down. My son was still fully clothed and had his DS in both hands, they were sitting maybe 3 feet from each other. I ordered my son back to his room and reiterated our open door policy as calmly as I could. I plan on talking to both later today after I've had time to collect my thoughts and calm down, and get the full story from both of them. I want to be careful with my wording & my response. I haven't said anything to my husband yet because he will assume the worst, that son is molesting daughter, and will blow a gasket. Obviously this is a very serious issue and I need help on how to deal with this. We've had talks about private areas, safe touch, etc with both of the kids so I am confident that they are both aware that it was unacceptable behavior. We don't allow them to be alone unsupervised (mainly due to bickering). I feel this is beyond "playing doctor". Advice please?? via /r/Parenting http://ift.tt/2CFHTbF