I'm assume they are together, so there are not legal custody orders set up. If this is the case, she is well within her legal rights to just pick the child up and leave.
My suggestion tho, would to get in contact with the police before hand and start the process of a AVO, that way they will be aware, also as soon as she leaves, start the custody.
Also before she leaves tell her to take some important documents from the house, birth cert etc, as she properly won't be able to get them afterwards.
I hope your sister is okay.


Love MN ;-)