Your sister is clearly the exception that proves the rule - one glance at the name registers shows that Grace simply was not a popular name choice for our generation, and there was a reason for it.
For what it's worth, my own name is exactly the same - a very cyclical name that goes in and out of fashion with a bang. I am also late 30s with the name, but I was virtually the only one with it when I was younger (never met another peron my age with it), because it was very unpopular in the 70s when I was given it.
Now - it's massively popular again (Isobel) and everyone goes on and on about how it's a 'timeless, classic' name - no it's not timeless, it's cyclical - I know, because I was massively teased for it when I was a child! 
People seemingly cannot get their heads around the fact that names that sound so lovely and fresh to our ears now, won't to the next generation, who'll think they sound dull and tired through sheer familiarity and over-use. Jack is a good example. Hardly anyone from our generation is called Jack (certainly not as a given name; maybe as a nickname). Another very cyclical name.
I'm not saying these aren't nice, solid, classic, old, established names. I'm simply saying they're not timeless and that they are susceptible to the vagaries of fashion, simply because they are so popular.
Every generation chooses a different set of names from their parents (otherwise we'd be calling our children Joanne, Claire and Karen). There will be exceptions of course, but these exceptions prove the rule.