Tia is really very cute. It's pretty tied to the seventies and eighties here but maybe not in other places. (It also means Auntie in Spanish, so obviously it's not something Spanish-speaking people use.)
I think it is better to have a less masculine name for a sister of a girl whose name is a direct, add-an-a feminization of a masculine name, especially if she goes by Theo or Teddy or some other boyish nn. To tell the truth, if I met sisters named, say, Theodora and Frances, nns Ted and Frankie, I'd wonder what their parents are really trying to do.
However, I would not choose Jemima. It's more cartooish than anything else, and in the US it's very much associated with Aunt Jemima pancake mix and syrup and with the now-very-offensive Mammy stereotype that went along with it.
You're entitled to your own opinions. You're not entitled to your own facts. -- Daniel Patrick Moynihan