Charlotte is the feminine form of Charles, so it's almost like naming them the same thing. I think it's too close, sorry. FWIW I really like both names.
ETA: What is your grandmother's middle name? Could you use that?
The names up for combination are Charlotte, Julia, Karen, and Felicia. I really dislike Karen and Felicia. We may just end up not naming after either of them, i feel like it needs to be both or neither.