I think you're way overthinking the naming process here. Just pick a name you would like her to have and that you feel is consistent with the names you already chose for her brother and sister. All the details you've given here are irrelevant to what her name is.
I mean, if I was going to go by the physical description you gave, (I didn't bother clicking the links) I would have said right off, Kurstyn. Because there is a little girl in my daughter's school named this and she is all freckly with the most cartoon-hideous orange curly hair you ever saw. But obviously Kurstyn would not be the kind of name that type of parents would choose for one of their daughters, not when they've got Penelope/Nell and Gratian
and maybe Theo
Just going by her family (and their ethnicity isn't that important either; nobody keeps track of 1/16ths of this and that) I'd say name her Charlotte and call her Charlie.