Weird child behavior?

Advice please!! I have a bonus daughter (8) and me and my husband have a 4 year old daughter together. The girls share a bedroom, and we have a camera baby monitor in there to check on our 4 year old because she likes to get up and play after she is supposed to be going to bed. Tonight the girls were in their room watching a movie and I told my husband to check if the movie was over because it was time for bed. He looks at the camera and my bonus daughter was tickling our 4 year olds butt. (and not in like a funny making her laugh way, like how you would tickle someone’s back) he called them down and asked my bonus daughter about this and her reaction made me super worried. Her face went pale and dropped. She turned away like she knew she was in trouble and wouldn’t look at my husband as he asked her about it. Bonus daughter said she was tickling her back but our 4 year old confirmed it was her butt (not that we needed confirmation since we saw it). I think the most telling thing is how my bonus daughter reacted when she knew we saw. She was like a deer in headlights and knew it was wrong. When my husband pressed more about what happened she started to cry and continue to lie. He took her aside to ask if anyone has done that to her and she said no. We let her know she wasn’t in trouble but it wasn’t okay. Did we handle this okay? Should we not have them share a bedroom?