Could sauropods sit?

Probably. Otherwise I think it would be difficult to lay eggs without injuring the shells.

Here we see a female Supersaurus laying an egg. The Supersaurus based on skeletal by Scott Hartman that I modified according to a figure 1 in Siegwarth et al (2011).

Edit: Changed settings so image size is larger. If you double magnify you can see it at a fairly big size.

Refs --

SIEGWARTH, J. D., SMITH, C. N. and REDMAN, P. D. (2011), An alternative sauropod physiology and cardiovascular system that eliminates high blood pressures. Lethaia, 44: 46–57. doi: 10.1111/j.1502-3931.2010.00221.x

Lovelace, David M.; Hartman, Scott A.; and Wahl, William R. (2007). "Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny". Arquivos do Museu Nacional 65 (4): 527–544.