I don't understand Jemma's dislike of Aaron to begin with, but it's bordering on pathological. We get it, you don't like him, but he's your sister's boyfriend now. It's tacky and tasteless.
The more I see of Jemma in the public eye, the more I get what Aaron was picking up on from her in the house. I always had the sense that he was trying to tiptoe around what he knew would be a big explosion, and it made Jemma crazy that he wouldn't allow her to go off on him.