She also has regressed to asking for help dressing and brushing teeth and whatnot.
Kids at this age tend to struggle emotionally with the transition from toddler to big kid. It's easy to be wistful about all the extra attention and closeness that results from being "babied." With your DD seeing it first-hand with her sibling, I would expect it to be worse.
My DD7 still loves it when DW and I suddenly pretend she's a helpless little baby... just as long as it's not in public. We go way over the top with it.
