What is Andesauridae

No scientific definition. Andesauridae was coined by Bonaparte and Coria with the discovery of Argentinosaurus in 1993 to house it plus the name-bearer Andesaurus and Epachthosaurus, based on the presence of a "hyposphene-hypantrum" complex (extra "joints" formed by a process on the backside of the vertebrae—the hyposphene, that fits in a depression in the front side of the next vertebrae—the hypantrum, to add rigidity and stability to the vertebral column) that other titanosaurs don't have.
However, hyposphene-hypantrum articulation is not unique to Andesauridae nor Sauropoda or even Dinosauria, Argentinosaurus doesn't seem to have this feature at all, Epachthosaurus may be a lithostrotian of some stamp, and Ruyangosaurus from China's Mangchuan Formation who tentatively joined them courtesy of Lü et al. in 2009 differs from "andesaurids" in at least 20 characters of its torso, thigh, and shin, and is probably a lognkosaurian.
As a family, Andesauridae wasn't well supported in the first place, but with Andesaurus now the sole member it's in tatters. Andesauridae is derived from "Andesaurus" (for the type specimen, the "Andes lizard") and the Latin "-idae" (denoting a family).