The rule is that you're allowed to share IC whatever information you, the player, know.
However, it's generally considered poor etiquette to simply instantly have Character A know stuff that Character B learns without any in-game communication channel open between them.
If you have Character B send Character A a letter with the information, Character A can share it however he likes.
If you roleplay Character A getting the information through the family grapevine in some reasonable manner, that's also pretty much fine.
There are, of course, some people who feel that there's no problem sharing whatever information with no explicit IC justification—and those who feel that sharing any information between your characters is a no-no.
And, naturally, if the information being shared is sensitive and will result in Character B's realm being at a disadvantage, you run all the normal risks of spying.
