Problems with my sister

So recently my sister has been running off to her boyfriend’s house 24/7, and she has refused to pay her part of bills, or she’ll scam you out of money. she also said she hoped my mom did have cancer so she would die first and then she could put my brother in a home. she then back tracked after I told my mom what she said and called me a liar. it’s gotten to the point where my mom has seen her true colors.

she‘s also dating guy with extreme manic disorder, bpd, and blood lust (she thinks it’s not that bad cuz my mom has bpd) hes also not medicated. he beat the crap out of his aunt at thanksgiving, and my sister laughed about it.

its gotten so bad my mom is ready to kick her out (she’s to nice to do it rn) and have me help work with my autistic brother. any advice?