Nope, the guardian is vulnerable to being killed. Otherwise how could the siblings win? They need to kill everyone EXCEPT themselves, including the guardian.
Well what if the guardian protects a wolf from the siblings killing him, and then that wolf kills the guardian, and the siblings kill him? I think the guardian would want to be protected from that, so he would choose himself!
A wolf can't attack another wolf. The siblings have to work like this:
Wolf sibling kills villagers that AREN'T SIBLING.
Villager and Wolf sibling try to get the villagers to vote to lynch the other wolves. Wolf sibling has to be careful, because saying "HEY MYKAEL IS A WOLF VOTE TO LYNCH HIM" will obviously tip off the other wolves as to who the sibling wolf wolf.
Wolves can't attack wolves, though, so they'd have to try to get wolf sibling lynched.
It makes the game more complex, but not really for the villagers. So as long as you're a villager, you don't really have to worry about it all that much.