I can't say that I know what to do about this problem but I think I know what it comes from. Do you know if this cat was weaned off its mama properly?
I grew up on a farm and when I was a teenager we had a mama kitty that died of distemper. She had a batch of kittens that was only about 1 month old. We found the kittens and bottle fed them until the vet told us they were old enough to learn to drink by themselves. They did eventually learn to drink by themselves but there were 2 cats in particular that I remember having weird habits after going through this.
One of the cats, who is still alive at age 13 and still does this EVERY day, will suck on one of his back feet to put himself to sleep. Another one of the cats would latch onto the shirt being worn by the person holding it and actually suck on the clothing.
Another thing I remember catching the above-mentioned cats (1 male, 1 female) doing was when they were about 2-1/2 months old we caught the female sucking on the male's "cathood," if you know what I mean!

Luckily, we just pulled her off of him and that never happened again. My guess is that she thought it was her mama.
Like I said, I really don't know what to do about it. Maybe a veterinarian could give you some advice. Good Luck!