I think it will depend on what is at stake, like if you are related to the hand. But I think it's okay to ask to show your hand, when this happens the dealer usually shows his hand to everyone.
I do not think it is rude, you can show it to one and all as they say in another comment above, but if I think it is despicable since you are benefiting only one person at the table, I take it almost as an illicit association haha
There is a rule at the table. Show one show all. I think you meant , is it rude to ask to seee a hand. I dont think so. I the player wants to its up to him. He shouldnt because it gives away info. thats why you ask.
When 2 players sitting next to each other are in a hand together and one of them shows his hand to the other player, without showing it to the rest of the table...is it bad etiquette to ask to see the hand?
I think it goes beyond that field and becomes a lack of common sense.
If the player didn't show it to everyone at the table, it's because he doesn't want anyone to know what he was holding.
Now asking him to show is an ignorance without scales.