Morgan Bartell, Ed.M., is a cat behavior expert and manager of behavior and training for the San Diego Humane Society and SPCA. She says it is somewhat common for cats to suck or nurse on things to comfort themselves. That could be themselves, other animals, people or even soft items such as bedding or clothing. She says cats tend to practice this behavior if they are orphaned or weaned early. (Ideally, kittens should stay with their mother and littermates until they are 10 to 12 weeks old. When they're separated earlier, they're likelier to develop this behavior.)
"When cats nurse on themselves, they often purr and knead at the same time," says Bartell. "This is often a self-rewarding behavior (meaning: it feels good for the cat), so it can be hard to stop. It is also something cats tend to do reassure themselves, either in stressful situations or just as a source of comfort, like a child sucking her thumb."
If your cat's suckling is limited to herself and causes no physical harm, there may be no reason to stop it. "While it might be annoying," says Bartell, "if the behavior is not causing any physical problems, it won't hurt your cat to continue. As she ages, your cat may even discontinue the suckling, though many cats continue for life."
However, Bartell warns that for some cats the behavior becomes a very serious habit that may become a response to any stressful situation, at which point it may be compared to human obsessive-compulsive disorder.
If your cat nurses to the point of significant hair loss, open sores or other body trauma, or if the cat starts eating items she sucks, it is time to intervene and you should seek the help of a veterinary behaviorist. These specialists are trained to treat compulsive or anxiety-based behaviors with medication. For less serious cases, you can take some steps that may not stop the behavior completely, but should help limit or discourage it.
First, try to identify what triggers the behavior. This may help determine whether the suckling is stress-related. If you can make a connection, then it may be possible to eliminate or at least manage the stress factor. "For example," says Bartell, "maybe the suckling happens more when children come to visit, or perhaps when another cat in the home is nearby, or when the cat hasn't received a lot of attention."
If the behavior seems stress-related, limit your cat's exposure to the stress. If visitors bother her, make sure she has a safe place to retreat when you have guests. Bartell also says spending more time with your cat, providing alternative activities and offering an enriched environment with interactive toys, climbing surfaces and more are all good ways to relieve kitty stress.
Perhaps most important, "Never punish a cat for this behavior," Bartell warns. "Punishment will only increase stress levels and may even intensify the suckling."
She advises you to evaluate whether or not you could be unknowingly reinforcing your cat's suckling. Do you pay lots of attention to your cat when she's doing this? If so, try ignoring her when she suckles. If she receives no external reinforcement, the cat may show less interest in the behavior.
Bartell says it may also be possible to transition your cat from nursing on herself to suckling on a substitute item, such as a blanket or teddy bear. Make sure you pick an item that won't shed pieces your cat could ingest. While this won't eliminate the behavior, it could prevent your cat from suckling herself to the point of trauma.
Please note, there have been cases of cats that actually eat blankets, socks and other items while suckling. This can be a life-threatening situation, so before you try this method, evaluate the level of your cat's suckling. If your cat is eating her own fur when sucking, do not use this method. Other warning signs to look for are holes starting to appear in blankets or clothing, missing items such as socks, chewed ends to curtains or towels, etc.
Lastly, if your cat tends to nurse on one part of her body, you can put something there that tastes unpleasant. Make sure it is something approved for use on cats! There are anti-chew formulas on the market. Be careful to place it only on the exact spots the cat suckles, so you don't discourage her normal grooming.
Read more: http://www.nctimes.com/lifestyles/columnists/theberge/article_0498ff4a-f0c1-5eba-8af2-4c0e52387c67.html#ixzz1m7y8IKoN
and on that note.... please tell your best friend to spay the mom cat too so she doesn't keep getting pregnant again and again 
