Do you have any "wiggle room" in terms of when you start up ? I'm thinking perhaps you could change your start time to 7:15 - that way if they show up a few minutes before you're open it's easier for you to just let it slide. I open at 7:30 - partly because I too had a family who would pretty consistantly show up about 7 minutes early. I was new at that point in time, and I really didn't want to get into a dust up with this Mum (she's the same one who would just "let herself in" as opposed to knocking) and this worked for me.