Possible duplicate of When are "if" and "whether" equivalent?. I'll risk saying that though the option with 'whether' is grammatically safer, the use of 'if' in its place here has grown so much that it's got to be considered acceptable in informal spoken contexts.
– Edwin AshworthJun 11 '19 at 13:40
Looks like the answers to (A) can be "yes" or "no", while the answers to (B) include "No, she will go with her sister."
The if is "binary" whether the "whether" seems open to many options.
– Juan LanusJun 11 '19 at 19:09
The if is "binary" whether the "whether" seems open to many options. – Juan Lanus Jun 11 '19 at 19:09