Squinty eye, no redness/discharge

My sister's cat has had this issue for some time now, a couple of years. She's not my cat, or she would have been to the vet ages ago. My family thinks it is nothing to worry about because it isn't weepy or anything. It is an on/off thing, but usually she has some degree of squinting, always in the same eye. There has never, ever been even the slightest discharge or redness to it. The only time she even seems to notice it, is when it is completely better, but heading toward bad again. She gets scratchy at it for a day, then she's squinting again the next, and the scratching generally stops after another day. What do you think it is? Maybe if I could give them more information on what could be going on, I could convince them to take her..

She hasn't been to the vet since she was spayed. I don't remember exactly when that was, but it's been awhile. They really don't tend to take pets to the vet unless they need to. At the time, the vet hadn't seemed concerned about it, and expected it to go away. He said to bring her back in a week or two if it did not, but, obviously, they never did.

She's done it again, recently. A few days ago, she scratched off/on a lot, was really squinty for less than a day, then moderately so. And now she's at the usual - just barely a difference, but it is kept a tiny bit more closed than the other eye. There's a little hair loss around it from the scratching, but other than that, everything looks normal.

My cat got scratched in the eye by a roommates cat when they were going at it one night.  I was away on business but when I returned he told me about it.  My cat wouldn't open his eye at the time, and they brought him directly to the vet.  There was a scratch on the lens.  The vet said in time it will heal up, best it can but that's it.  I got fancy eye drops and a return visit date a month later.  They put clear eye drops in his eye that glow in blacklight.  A smaller scratch still remained. 

Some times he'll lower that eye and squint a bit with the other wide open. He's not in pain, no discharge, no redness but sometimes I think it either gives him eye fatigue (looking past the scratch) or just bothers him time to time. 

Answer (in my not vet opinion):  An old scratch bothers him.