As long as they don't sleep together is what counts.
She only comes during the summer so she sleeps in his pull out bed. She was asleep when he did this
Given the age of your kids, I highly doubt that there was any sexual intent. However, I think you need to sit down with both and teach the importance of respecting each others boundaries. Additionally, you need to discipline this behavior if it ever occurs again. Finally, you may want to ask them where they learned this behavior and take appropriate action given their response.
I'm assuming that they sleep in their own bed in their own bedroom. If not, this needs to happen to prevent this behavior from repeating.
My son is 4 and his stepsister is 5. I went and checked on the after i put them to bed and my son had pulled up her nightgown and pulled down her panties. He was rubbing her butt cheek. When i asked him why he told me he didnt know what to say