"The girls visit their grandmother."
Translation:Las niñas visitan a su abuela.
Here "sus" is not agreeing with "girls." The possessives always agree with whatever is possessed, not with the possessor.
"Su abuela" can mean "his, her, your (usted), their, or your (ustedes)," but it has to be singular because "abuela" is singular.
It's the same way that in "mis libros" the plural "mis" is used because "libros" is plural, even though it is referring to only one person.
Thank you. That makes a lot of sense. The libros example was helpful. Have a lingot or 2 :)
I hesitated for a while, and I put su instead of sus, and this was correct, and I am happy. su is adjective, and its form changes with the noun it modifies.