What to do with psycho 3 yr old?

My aunt takes care of me (13) my younger brother (7) and her daughter (14 months) and our PSYCHO COUSIN i have no idea what to do with her if my little bro tries playing with something she goes and bothers him and constantly shouts screams and often throws tantrums and has hits all of us at LEAST 15 times a day my aunt is a complete and total pushover and often makes things worse by giving in after a so called "punishment" my aunt will "spank" her and she will often laugh after. my aunt takes her out of time out early and like i said will give her whatever she wants afterwards im SICK and TIRED of her screaming and my aunts pushovery ness i want a strong discipline tool or trick from any one PLEASE HELP