TWIN sister homework disaster?

My twin sister always slacks of on EVERYTHING. Instead of starting her homework (like our mom says 2) when she gets home she goes to her computer and watches a movie. Then at like 10 pm she finally starts her TON of homework. Then we can't watch our family show because we cant't watch without her... This makes my mom REALLY mad, and that makes her in a bad mood.... What should i do? i don't want to be a tattle tale, but i want my sister to do her homework so we could actually DO SOMETHING after school... WHAT DO I DO?