Написать записку своему другу и объяснить, почему вы не можете прийти"

Ann, sorry, but I will not be able to come to your party on Sunday. My mother asked me to look after my little sister because mum must go on a business trip. I am very sorry that  I will not be able to come. I hope that you will understand me. Suppose, I will visit you next week when my mother returns. 

Best wisher,Julia.