Tell them that they are paying more attention to their new child then you and are blind to your needs. Your sister does not need a god damned "toy room", she can use her own room. Or, if you have an empty closet, put her toys there and take them out for her when she pleases.
You are kind of a dope, I admit, but things like this make me frustrated at the unfairness.
Good thing I am the youngest child.