I Broke Mom's New Phone

Yesterday I was playing a racing game on my friend’s tablet. My sister walked in and told us to keep it down. She made me give back the tablet, but I had one more race planned. So... I borrowed mom's new phone instead. Of course, I dropped it and cracked the screen. Now I’m grounded forever. Should I tell her or hide the damage? Is it okay to keep a secret this big? What would you do?