Oh honey ... these silly cats don't know when or how to say no and mean it. My Pearl kitten (now over 2 years old) will still nurse on her Momma Lexus and Lex actually lets her do it. Pearl really isn't sucking, she's just drooling all over Lex's belly and makes a horrible mess, but it it so blooming cute that I just don't have the heart to make her stop.
I've caught them going to Shareena when their mom walks away. She's my first kitty who has never had kittens, but she thinks these babies are hers. She actually lets them nurse, just licks them to death while they're doing it.