What to do about a lie gone bad?

My Sister and I were fighting and she threw my iPod so I threw her's at her and I didn't mean to throw it at her face but it made a gash on her eyebrow and we had to go to the hospital for her to get stitches and we both agreed to tell our mom that she just hit her head on the metal cabinet the we all have got hit by and now I think she suspects something and I dont want to tell her cuz I know that I will get in even more trouble for causing her to have to get stitches........so what so I do because either way I will get in trouble