For some reason, novelists in 18th century England often used the issue of incest as a plot device. Usually the incest was a near-miss. Siblings would be separated at birth and find out at the last minute that they were brother and sister or two people who think they might be siblings find out that the real father or mother is not who they thought them to be.

In Tom Jones, when Partridge finds out that the woman called Mrs. Waters is really Jenny Jones, in horror he reveals to Tom that she is his mother. Tom believes he has had an affair with his own mother!

The eNotes study guide states that the " overarching theme of Tom Jones is virtue and vice" and that "Tom’s imprudence often manifests in his behavior with women." In other words, his promiscuity has led him to commit what some consider to be the greatest sexual indiscretion. Fielding is teaching us to be prudent, or wise, in our actions. Tom learns this lesson by the end of the novel and decides to change his ways.  

I'm not quite sure what you mean by "social dimensions of incest." Even though, or maybe the reason why, it was a frequent theme, incest was and still is a social taboo.