I expect I.A to move by itself -- without I.B. Of course, in doing so, I.A will need to be renamed to II, and what was II and III need to become III and IV. This way, if the user wants to move I.B along with it, she can always select both I.A and I.B together and then move them.
To play devil's advocate for a second, if you opt for the other approach where moving I.A automatically moves I.B with it, then how would you address the case where the user really does want to move I.A by itself? I have seen a program (can't recall which, perhaps Word 199x in Outline mode) trying to address this by having all siblings move to the left (I.A and I.B) but only the selected one move to the right, but I've always felt that it was a case of a program trying to be too clever -- and becoming irritating to use in the process.