So I’ve given a couple of answers to variations of this here and here. The important thing to remember, really, is that Shikako is playing a high stakes game of ninja chess for basically her entire life – another reincarnated person is going to be a wild card. And she’s really not going to like or trust that.
If you picture her suspicion and distrust of Sai when he showed up early on, and then realise exactly how much she knows about the inside of his head and how many strings to pull and what his goals/backstory is and who he reports to… and then realise a reincarnated person is going to be a giant blank spot who potentially holds some serious blackmail material over her if they realise she is also a reincarnation…. yeah.
Even if they mean well – and there’s a good chance they won’t, remember – they’re still coming very close to what might be her most viciously defended secret. It’s not going to be pretty, is what I’m saying.
-Silver Queen