Bobby, you're right as rain. I totally blew this! On IMDB, I saw Ann Morgan Guilbert (Millie Helper on Dick Van Dyke) listed in the TAGS episode "Aunt Bee's Cousin' and my mind read it as her playing Aunt Bee's cousin Ella. Shame, shame, shame! Serves me right for not confirming it on the DVD. Sorry folks.
Completely forget about this one. Aunt Bee's niece comes around in one of the color episodes to leave her little baby with the Taylor's for a week while she and her husband go to a wedding. I feel like I remember the wedding was really far away, but don't recall where. I also don't recall the niece's name - maybe Mary?? But Aunt Bee was afraid the baby hated her because it cried every time she held it.
Nice work Phil! Forgot about that one too.