Awkward situation....?

My bestfriend and i arranged to have a sleepover this weekend (friday to saturday) at hers. But today is her older sisters birthday and we're all going to hers. So I was supposed to have been there at 2:00 but i lost track of time and she called me at 2:30 to ask where i was and say that they're waiting at her house (pretty much her whole family) for me to get there and then they're going back to her sisters. I'm not being funny but ill just feel awkward at her sisters birthday with all her family. I don't really wanna go yet:/ Should I just call her and say i've gotta tidy my room before i can go and tell her to go back to her sisters and then i'll go straight to her sisters? I know it sounds bad:/ what should i do?