Are you in touch with child psychology services? That would be something to ask for. Without knowing what the behavioural issues are, it would be unusual and difficult for a primary school to go straight to expulsion. The statementing process ought to have as at least one of its results that your niece is provided with a teaching assistant's time and support, so that at key times she can continue learning away from the main class but not be able to upset others and wind herself up. So this might not be a bad thing. Hopefully she now has a stable home background and over time her behaviour will hopefully settle down.