My relative was at uni in Glasgow, but finished about a month ago. Then they came to stay with my other relative in Inverness, so broke lockdown rules to travel there. 
All has been fine for a month, two relatives living together.
However, the relative they are now living with has decided to travel to England, again breaking lockdown rules.
To visit a partner.
The relative that moved in with them from Uni has now asked me if they can move in with me for a while, whilst the relative self isolates when they come back from England.
I don't want to, but need help wording a message to explain why not? I don't want to fall out with them over it all, but also don't want someone living with me for weeks. And I'm not happy about the other relative breaking lockdown again.
I have a husband and two little children.
If it's relevant, the relative only texts me when they need something. 😩
Thanks for the help