First off, does your sister have anymore of the cream left? If so, tell her to start putting it on the paw right away. If not, if she brought her cat to the vet the first time for this, I don't see why she can't bring her cat this time also.
I feel sorry for your sister's cat.
Secondly, may I ask why your sister got the cat in the first place if she doesn't pay attention to it. Is it because the cat is 14 years old and doesn't play like it use too when it was younger. Out of respect for you KYS, I'm not going to say anything more on this matter as I'm sure you must love your sister dearly.

Please try and convince your sister to have the paw looked after by a vet as I have no clue as to why her cat is doing this. She should be blessed to have one of God's most precious creatures.

Next time KYS, tell her to get a gold fish.