HOW DO I CONFRONT MY MOM ABOUT CHEATING ON MY STEPDAD?

I MOVED TO YEARS AGO TO A DIIFERNT COUTY WITH MY BF ,EVERYTIME I GO HOME TO VISIT SHE DOSENT WANT TO SPEND TIME WITH ME.SHE GOES OUT WITH HER BF AND STAYS THE WEEKEND WITH HIM,SHE HAS TOLD MY STEPDAD ITS JUST A GIRL FRIEND BUT THATS BS.I HAVE HAD FAMILY MEMBER TELL ME SHE IS CHEATING.I HAVE A NINE YEAR OLD LITTLE SISTER AT HOME.I DONT THINK SHE SHOULD HAVE TO DEAL WITH HER GOING AWAY EVERY WEEKEND.ITS BECOMING OLD AND I HONESTLY DONT EVEN CARE TO COME HOME.MY STEPDAD DOES KNOW,AND HES LIKE IN DENIAL.