I think the GMs can make the difference between someone who's
1- at seals with 3 afk character in the little hut thingy (a priest, a linker and a professor) none of which are moving or reacting when I try to get their attention
2- partying with a priest, be it in the same house or not.
If your sister is playing with you that's fine, I got no problem with that.
Basically the rule is you can only play 1 character at a time per physical person.
I've played for 2 years on private servers which did not have dual clienting, thousands of people play on iro and kro. If they can do it, I'm sure everyone here can too