need some help

ok my sisters boyfriend cheated on her with her best friend right n shes upset but shes actually gona 4give em but now her other best friends boyfriend is asking my sister out.... my sis knows how much her friend likes him shes putting her friend first but i can tell shes falling for him she keeps going on bout how he called her hot but at skool he tells me that my sisters ugly and shes weird i dont no wat changed n i dont no if i should tell her but im worried help i need advice about wat 2 do