What would you make of an ex friend who is trying to befriend your family on Facebook?

[Mrs. Muffintop Rules My House]

I used to be friends with this girl whom I treated her like a sister but began telling lies about me in our circle because my cheating arse of an ex got her involved into our final squabble that ended both relationships for good. Him because of his philandering eyes and her because of her mouth and lies.

I unfriended and blocked her from my Facebook. During thanksgiving while my extended family was visiting, I noticed a friend request from her to my sister in her inbox. The girl knew my sister through me barely and they are hardly what you would call acquaintances. Why is she befriending my family? She is still friend with my ex whom i don't communicate with anymore but only if it's concerning our child together ( boundary issues, he was trying to sleep with me after he remarries). I feel as if the girl is befriending my family to keep tab on me. Am I overreacting or am I correct in assuming that she is stalking me?
My sister was using my iPad and asked if I knew someone by that name because she had gotten a friend request from her.

[jay_loya2000]

block her

[chris b]

...and how did YOU get into your sister's account to even know what was in her inbox???