Kate's reaction towards Peg's comment about ''baby brain'' is a normal extension of ''familiarity .''
Familiarity is a taboo concept for everyone that didn't stand the tests of time.
Peg was, is , and always will be, a stranger, the fact that she's marrying Harry , becoming SIL, living there was irrelevant .
Only time removes the taboo of familiarity .
Also Peg is intrusive and the stereotype of a person whose body language mimics ''openness '' and ''closeness /approachability''
Even if she was the nicest person on the planet for certain people it makes her untrustworthy a ''aiming to fit in'' which (irrelevant, but even in my case) makes her unlikable .