Your sister shows obvious psychological impaiments, shopping and singing along with the music is something like standing on the street corner and talking to yourself.
She knew she was going over the edge, wanted to get away, and you go and point it out.
Singing and dancing to the Muzak in a retail outlet, DSM 0.0219.
Gifting to try to overcome the feeling of out of control bra spending. DSM 0.0416.
Fear of being grabbed by the hair by an Orca playing Barbie. DSM 0.0630.1.
Tolerance and acceptance of behavior will not help, or make it worse.
The proper response is Un hu, Ya, and the universal, OK, without the sarcastic or questioning tone.
Sister is the most unguarded of mental states, the subject may say or do anything, and expect to be kept as a family member. Consider it.