My sister ditched me for her boyfriend.jst cz i told hr he's nt the perfect guy for her.wt shud i do ?

v only hv our mthr.she's nt going to like this relationship.i dnt think my sis shud hurt my mum.This guy hs told me he wud hv gone out with me if he met me b4 my sis.he's rly strict om my sis.he tells bad abt my sis to me.wen i tel them to my sis she takes his syd and scolds me.she dnt undrstand tht i say things cz i luv hr.i dnt wnt anything bad hpning to her.she's only a year older than me.im tryin my bst to be the bst sis to hr.bt i get ditched evn for her frnds.i dnt mind her being mean to me but i dont want her to hurt my mum and i dnt wnt her to ruin her life.. please help me..