The thing linking the uncle to Sara's abuse is the fact that his DNA was found on both ends of the cricket bat and belt which also had Sara and US's DNA on both ends. The cricket bat fit a number of old and new injuries so it is highly probable that the uncle beat Sara as well. There is no other logical explanation for his DNA being found on both items. The prosecution also said that both the uncle and BB would message or call US to tell him of Sara's misbehaving knowing that he would come back and beat her.
He's as guilty as BB in my opinion even though US tried to shield him by saying that he had knowledge of what happened to Sara. That's impossible living in such a small house. It was said during US's evidence that at one point FM had offered to take the blame for everything; it's not clear to me at what point he did this and how the prosecution know. He might get a lower sentence simply because he wasn't in the same position of parental responsibility, but I would be very surprised, and disappointed, if he isn't found guilty of murder along with BB.