Two questions

I have a couple of questions about my kitties. Some of you may remember me posting about Taz & Cuddles.....their mom was a pregnant stray. Well she found a home (and so did the female kitten) and then our oldest cat Pepper passed away, so we decided to keep Taz & Cuddles. They are almost 14 weeks old now. They try constantly to nurse on Isabella, who is now the oldest at 16 months old. It is mostly Cuddles who does it, and he was the slowest to develop. He did not start eating on his own until almost 8 weeks, and was just the slowest to develop in every aspect. But occasionally Taz will also try to nurse. They suck on Isabella until she is raw, but she just lays there and lets them do it.....she even grooms them while they are doing it. Is there any way to stop this behavior?
Also Cuddles is very un social towards people. Both have been handled constantly since birth and have been around tons of people and other animals. Taz will jump in our laps, rub up against us and loves to be held.....and he purrs ALL THE TIME. But Cuddles cannot stand to be held, he runs away, will only come to us when it's time to eat or very rarely he will play with one of us. The only time he purrs is when he's trying to nurse on Isabella.....but NEVER purrs for us humans. Any suggestions on how to make him more social? Or will he always be this way?

Some cats do not like to be held or given pets so you just leave them be and let them be cats. I would buy a bottle of Bitter Apple and start applying this daily to the nipples and tummy of Isabella. They can really harm her if you let them continue to nurse on her- even though she is not giving milk, they can open her up to bacterial infections. It is just not a good idea to let kittens on solid food nurse anymore.