Cat third eyelid

Help my 6 year old male cat has heart disease which he has treatment for and is fine, a couple of weeks back he was sneezing and coughing I took him to the vets who said his throat was a bit red but this the was fine as he was eating and drinking as normal. Then his third eyelid appeared and he had watery eyes, another trip to the vets who said they was unsure what was wrong with him and that he could have hurt his back and gave me anti inflammatory drugs but to not use them if he was sick, and of course he was sick. His eyelids are still there but seems fine he's a very chilled out cat due to his heart, what worrying me now is his sister now has watery eyes and her third eyelid is starting to show??? I really don't want to go back to the vets to pay for them to guess what the problem is, any advice would be gladly appreciated

Sounds like a little upper respiratory infection, which can be contagious.  Keep an eye on her and if her symptoms worsen, then take her to the vet.  Look for increased lethargy, decrease in appetite, change in litter box or play habits, and any change in color of any eye or nasal discharge.  If you've got or can get some L-Lysine tablets from the store, that might help.  Get the 500mg tablets, and crush into wet food twice a day.  You can give to both kitties.

Ok thank you I will get some tomorrow, and yes other than that they are both perfectly normal

Great advice from @stephanietx.

It's good that you are conscious of your cats, the inner lid I always say is like a window to their insides, it is sometimes the only sign they will give us that says, something is not right.

In my opinion, a full course of clavamox liquid for both, some l-lysine supplementing, and you should def. see those inner lids disappear.

If not, there is still something not right.

I know this can be frustrating, I've dealt with this more than I can count having a cat rescue.