I'm going to say the same thing I said with someone else who asked to have poison ivy as a mother.

Phoebe is 21. As far as she knows she is an only child. I honestly don't know how it would work if poison ivy had another kid, especially younger, but I do think it could provide an interesting story line.

Actually, I've had an idea if you do continue with having Poison ivy as your characters mother. 

I'm happy with it.

On a side note, I'll start gym soon if everyone wants me too