You never really find out how many siblings he has, but I think you can infer that he must have some based on some of the little bits of narration you get in the books from time to time, particularly on the subject of the frenzied nature of mealtimes, when everyone basically had to eat as fast as possible to make sure they got their fair of the food.
Just as it's interesting how many DW men are bachelors, it's also interesting how many of them don't seem to have siblings (or those we know about). Vetinari, Moist, Nobby, Brutha, Mort, Colon, Detritus, and Carrot have little tidbits of family background thrown out there, but there's no conclusive evidence that any of them have siblings. Whereas, most of the key female characters--Granny Weatherwax, Tiffany, Esk, Angua, Cheery--not only have siblings, but one or more of them plays an important role in defining who they are.