Maybe her head's spinning trying to keep track of all the secrets flying around?
Or perhaps Kathy asks her to keep quiet so she has a chance to break the news to Ian herself?
Talking about Sharon's comment that Ian's like a brother to her: remember the DNA test they took that was corrupt and didn't provide an answer, and given the very popular theory that Gavin is Sharon's real father - DTC wouldn't retcon Gavin as Ian's father too...would he????!!!!!