I want to get custody of my nieces even if only temporary, or at least guardianship of both my sister and her daughters

My 35 yr old sister is mentally disabled (mentality of a teenager or less) and has 2 daughters 1 and 5 years of age , I have tried effortlessly to assist her in raising them properly but I am getting no where. Her boyfriend 47 yrs old (which is the father of both girls) Has failed to keep a job for more than a month and is living solely off of my sisters disability checks from social security. Needless to say they struggle every month to pay bills and keep food in the house (that is where I come in) He is overly aggressive with the 5 yr old when he is irritated due to the fact he cant even afford his own bad habits (which is smoking and chewing tobacco) He mentally abuses my sister by telling her if she kicks him out he will take the girls etc.. I need to know what I am legally able to do please.