The dealings I've had with Symantec licenses in the past have all been valid in the "Country of Purchase".
If you think about it, you could install one of the three on a laptop - then get your updates from just about any IP Address. The licenses are usually activated during, or just after, the install routine. Once the three have been used up, no more installs will be allowed.
Our office licenses allow to us to install on in-office machines, employee's home machines and employee or office laptops.
So you should be OK installing on your S-i-L's machine as one of the three.
K