I can identify. I have a mother and kitten, her other two kittens were rehomed together elsewhere. The kitten I had was a real mummies-girl, hence keeping them together.
She wouldn't stop suckling, and the mum didn't seem too bothered about it. I spoke to the vet, and he said to leave them to it, that her milk would gradually dry up, and it was more for comfort than a food source.
She has stopped now, but she carried on trying less-and-less frequently until she was about 6months!
It's not a real problem, unless you want to re-home the kittens. If they're eating real food and drinking water, then I'd just leave them to it.