why is my mom always mean to us?

ok my moms 43 and im 18 and sister 21 in like few days,, anyway she has been being awful to me mostly but can be to my sister. we both have mental illness and she yells at us about that, yells at me about my eating disorder, how i act and says im an user and liar and etc. she gets on my sister for being autistic and yells at her about being fat, yells at me being an pig etc. its dumb. we tell her she has bipolar or something but says no i dont. she threatens to kick us both out and get us an small apartment where we can let things pile up and figure our own f in way out. shes not always this way but alot. im tired of it. ive learned to keep my mouth shut most of the time but at times it gets me and i cant just take it . it hurts me to hear the crap and right now is going off. threatens to tell my bf lies. she accuses me of stealing alot when in fact i dont. she just cussed now saying im a back stabber and bs. im tired of it. what should i do? what ?? someone help and why is she this way?