This is the best option. I don't understand one part of it but after looking more into this I think I do.
The stepdaughter with a child would come to the US leaving the child in her home country. After entering the US then She would basically file for I485 to adjust her status and also apply for I130 based on this from the USCIS website together.
"Concurrent filing is always allowed for all immediate relatives of a United States citizen because there are no numeric limitations in this category."
So she would file I485, I130, and I131 all at the same time once she entered the US. Once the I131 was approved she could return to her home country to care for the baby and await her I485 to be processed.