It could well be that Danielle C and Isabella are sisters. But is there anybody out there who really knows? The fact that they have the same last name doesn't mean a lot, e.g. napodiume (a russian modeling site) currently lists 30 models with that name.
Back in 2003 Met introduced her as Andrea C's sister. Here's the cover. Again I'm sceptic because according to the info that I have (old model pages from modeling agencies), they have different last names.
The Metgirls covers from 2003 do definitely not show Danielle C (I dare to say). - Is she IN that sets, or what is the reason for showing the covers here?