My sister attended a family party in a tank top that visibly showed a very large tattoo, the Chinese symbol for horse, on her back. She smiled, "I can't wait to shock Grandma! She's going to be so upset." It was definitely said in a "in-your-face" way.
Granmda arrived, saw my sister and asked, "Anna, what's that say on your back?"
Sister smiled, "It's Chinese for horse."
Grandma replied, "oh, I thought it said Kick Me."
Grandma laughed. Sister didn't.
Go Grandma!
Why "Go Grandma"? Unless I'm seriously overlooking something in this story, the grandmother was the only one rude during that little conversation.

The sequence I'm reading is:
1) Sister gets a tattoo and thinks Grandma will disapprove (and it seems she was quite correct). -- Not rude; her tattoos are her own business, not Grandma's.
2) Sister makes sure the tattoo is visible for a family party, with the intention of seeing Grandma's reaction. -- Arguably rude. Trying to "shock" someone and thus provoke a confrontation at a family party is rude or at least quite inconsiderate to the family. However, wearing an otherwise appropriate outfit that shows a non-obscene tattoo to a party does not seem at all rude. Therefore, the only reason I would say Sister was rude is because she stated she was trying to shock Grandma,
which Grandma presumably didn't know about when this conversation happened.3) Grandma asks what the tattoo says and is answered. -- Either party
could be rude or perfectly polite, depending on tone, which was not described.
4) Grandma makes a comment about thinking it said "Kick Me" -- Rude. I really can't really think of any interpretation of this that isn't insulting (most likely implying that she was too stupid to check what it meant before having it tattooed).
My interpretation: Sister was rude by trying to shock Grandma/provoke an uncomfortable confrontation in the middle of the family party. Grandma was even ruder by actually
starting that confrontation and making nasty comments about her granddaughter's quite non-offensive tattoo.
If I've missed something, can someone please explain why Grandma deserves cheering?