Please accept my condolences in the loss of your beloved mother. May she rest in eternal peace.
Her husband is her legal next of kin. If she died without a will, everything goes to her next of kin. In this case, her husband.
You and sissy loved mother. Unfortunately, love means little in a court of law. Her husband doesn't have to allow anyone into their home. His marriage so quickly after mom's death may not be right in your eyes, but it's legal.
I wish you peace. But, you have no legal basis for complain. His bless.