Power of Attorney is a document that has effect only while the person who executed the document is still alive. So, once your mother died, the disposition of her property
depends entirely on her will (assuming there is a valid will). Even if the will is invalid, the law in all states would divide her estate equally among her three surviving children, just as her will does. The fact that your sister had power of attorney while your mother is alive no longer has any bearing on how your mother's estate is to be distributed.
As executor of your mother's estate, appointed under the will, your sister's job is to distribute your mother's estate exactly as your mother instructed in the will. It does not give her any extra claim on the estate itself, and does not allow her to change the distribution set out by your mother. If the will says that the estate is to be distributed equally among the three siblings, that is what your sister is obligated to do. The provision regarding your sister's husband applies only to your sister's share and has nothing to do with the share that you and your other sibling are to receive.