I think that what you saw was a T shirt saying this: The World's Okayest Sister The word 'okayest' doesn't really exist, but the T shirt is a joke based on the more common T-shirt 'The World's Best Sister'. It's a big compliment to tell your sister that she is the best one in the world. If you are 'an okay sister', it means that you are just about all right, acceptable, not great... and it's funny to say that someone is the 'most okay' sister in the world.

I think it means the best sister or the coolest sister.

I think this iş a joke This isparomasis ıt sounds ilke wold o gayest sister.