Is it normal for 14-week-old sibling cats to still suckle on their mother (video)?

I don't think it is particularly normal. Normally domestic cats are weaned at about 8 weeks of age. So 14 weeks is about twice the normal time. But apparently, free-living kittens often remain with their mother for 12 weeks or longer. And there has to be some flexibility in how long kittens are weaned. It seems to me that these sub-adult siblings are seeking comfort from being nursed by their mother. And that said, there must be a background reason why they are seeking this form of emotional comfort. Might it be fair to say that they are slightly stressed for whatever reason?

I don't think that it is anything to get really concerned about. The prediction would be that the mother will eventually push them away but she appears to accept it. As all three of the cats accept what they're doing it should be regarded as acceptable by their human caregiver. It's just a question as to how long it goes on for.

You wonder whether there will be any behavioural problems in the sub-adult cats when they grow up and become fully adult. Probably not. And you wonder whether the mother is producing milk in this video. Certainly not I'd have thought and therefore the siblings are not suckling on their mother's breast for milk but solely for emotional comfort. The mama cat will likely stop lactating by the eighth week after the kittens are born.