I guess that depends on how obvious that it was that the other person was reincarnated. Because there’s a lot of room between the lines for siblings to exist (Chiyako doesn’t, for one and we know next to nothing about the families of other rookies).
So the first part of the question ‘what if Konoha 12 had siblings’ gets a very non-reaction because it’s not a big deal. (Unless, you know, it was someone who’s family life probably should have rated a mention, like Sasuke or Naruto.)
The second part of the question is more ‘how would she react to another SI’. And, lets be honest, that would probably not be great. There’s a lot of danger in the Naruto world in having dangerous secrets – and more in other people knowing you have them. Shikako tries to minimize her personal risk by keeping her head down, especially early on, and you can never be totally convinced that someone else is going to follow through with a plan like that. Or do it well enough to not get caught. Or not spill the beans about you if they are.
See what I’m saying?
In summary, would probably come down to who that SI was, and what they were like and how they acted – and whether they identified her as out-of-place or not.
-Silver Queen