I need help with my sis and nephew?

My sister just moved in wit me my son n bf frm outa town, I love her but she lets her son get away with everything. He spits on my floor, screams, cries if hes not getting his way, hits her, yells at her, bits her, drives me up the wall because he doesnt listen. She lets me put him timeout but the second he comes out she lets him stay out. When he acts up she sayd everytime "Are your gonna stop?" and he says yes but duh hello of course hes gna do it again. Hes 3yrs old and knows how to use the bath room but he pees on his hands all over the toilet and my floors now I know when kids are being potty trained they might make a mess but he knows how and she doesnt go to the bathroom wit him and show him or whatever she has to do to not have him do that anymore., She says he acts the way he does because shes a single mom and he doesnt have a dad to tell him what to do which I think is bull. Shes says I shouldnt talk **** cuz I dont know how it is to be a single mom. But that dad thing she says is to me no excuse. Please help me figure out what to do I have already talk to her bout this but it doesnt go through to her shes 1 of ..those that shes always right even when shes wrong. Im getting to the pion to I dnt want her here anymore and I dnt want that to happen