The uncle is the inspector's uncle, the wife's uncle-in-law, and the son and the niece's great-uncle.
He becomes sick if without food for 3 days, making him the most resilient to hunger in the inspector's family. He also gets sick if without heat for 3 days, and his sickness threshold is also 3. Just like everyone else but the inspector, he becomes hungry, cold, sick, very sick, and possibly dies.
The uncle provides no gameplay benefits whatsoever.