Dear Mary,
Hello.This is Susan.I am sorry that I can't visit you next week.My parents are going out for work next week and won't be back till next month.So I will have to stay at home to look after my little sister.I don't think I'll have time to go to see you.Mary,how about next month?I would like to visit you next month?Will you be free?Looking forward to your reply.
Regards
Yours
Susan