1. "She resembles your sister more than would be required for a passing grade."
If there was a test to look like your sister, if someone resembled your sister enough to pass, they would have a "passing resemblance". If they looked even more like you sister than required to pass, they would "bear more than a passing resemblance to her".
2. "She looks more like your sister than one would judge from a mere passing glance".
PS: I've always thought of it in terms of 2., but now I think 1. is right.