I know Cam and Seely had something going on around when she first showed up, and I'm pretty sure they did before the show took place.
They never told anyone in the Jeffersonian about it to my knowledge, and ended it just as soon as it seemed to start.
This was around Aliens in a Spaceship, as they were having a secret lunch when they found about about Brennan and Hodgins being taken.
They had to pretend to be together in the Intern in the Incinerator for her fathers sixtieth birthday, long after they stopped ...whatever the title is.
Sweets, on the other hand, was never explained.
He was adopted at a young age by an older couple at 6 I believe.
We learn his mother was a carnival psychic, and his adoptive parents died within weeks of eachother, shortly before he began working with Bones and Booth.
I hope I helped a bit, anything else I don't know someone else may be able to clarify