Long time lurker but needed to post.
People who are in her house at separate times are NOT close contacts of each other. If Grace becomes positive they are all her contacts, and if one visitor becomes positive grace will be a contact but the other people are not. I'm literally just finished being a close contact of someone today.
To be a close contact you must be in direct contact and within 2 metres of a positive case for more than 15 minutes.
If I get on a bus 10 mins after someone who is positive gets off, I am not a close contact. I wouldn't even qualify as a casual contact in that instance
And to top it off its only the people who you were in contact with 48 hours before symptoms started. So come Wednesday, grace is none of their contact if they develop symptoms
Also the one household visitors, are supposed to be one household for the duration of the lockdown. So sisters might pick each other, or friends, but they don't have many different households at separate times. That should have been more clearly explained by Mehole