Of those two names, with no other information (surname, brothers' names) I'd cast my vote for Gwendolyn.
Why? Because Guinevere is very similar to Jenivieve (letters, rhythm) and, personally, I prefer that the names of sisters be a bit more differentiated. So I think Gwendolyn would be a better fit.
As far as middles go, my favorites with Gwendolyn are Elise, Claire and Elizabeth (both not Alizabeth--I think that "A" makes the name look silly).
Last edited by Nancy - Baby Names Editor; 08/14/11 04:54 PM.