Is it against the "rules" to share your key with your siblings?

[Sicarius]

Me and my sister joined a server and the host said its "illegal" to share your key with your siblings.

[Ad Bot]

[Sulfate]

if you are on 2 different computers in the same house - it's fine

if she is in a different house - the IP may jump back and forth, leading to possible deactivation

[Headcrab Zombie]

Q: Can you use one key for multiple people?
A:
One key can be used for any number of people as long as they are using the same name and are at the same IP address (ie in the same house).

Clan tags can be used to distinguish different people with the same name.

[DontCare4Free]

if you are on 2 different computers in the same house - it's fine

if she is in a different house - the IP may jump back and forth, leading to possible deactivation

That would cause authentication errors.
I have tried making my IP jump back and forth with a huge physical distance between them and that caused me quite some auth errors (VPN from vacation on a crappy connection).