You've already told her you'd prefer not to sit with her, right?

If she tries to sit with you again, just kindly remind her.

"W, I'm not being mean, for my own needs, I prefer you to not sit with me. There's a seat over there."

Use your own words, of course. Don't feel badly about it. Just respect YOUR boundary, and insist she do the same.

Me: 48 H: 50 - Married 21 - 3-S: 29,19,19 2-D: 27,26
BD: 08/2015 - D filed & OW disc: 09/2015

"Surrender to What Is, Let Go of What Was, Have Faith in What Will Be." -S Ricotti