I think it is totally normal. I have a mother cat and 4 of her 'children' (who are now over 1 yr old).
In my experience, you are witnessing the natural weaning process. My mother cat allowed nursing up until around 4 months and then gradually would not allow it...and that came in the manner of growling and sometimes swatting them away. It just depended on her mood and I am sure mother cats vary in willingness to continue nursing (for comfort). They were quite persistent at times and she was often seen running away from them as they tried to nurse, LOL.
Interestingly, to me at least, I observed that this was followed by a period of a few months where she would not sleep with them at all and only groomed them occasionally and on her own terms. And if they tried to nurse, she would back away again. Then, a few months later, she started becoming more affectionate again with the two female kittens and was more willing to sleep with them and be affectionate with them. During this process though, she never really lost her protective instinct and would intervene whenever her babies roughhousing went too far (in her opinion, I guess). Or if she perceived a threat from our other cat. It was interesting that she seemed to steer clear of the 2 male kittens and even now will interact affectionately more with the girls than the boys. (she is still protective of all of them, but lets them fight their own battles for the most part)
My theory is that she sensed the boys could present a threat as they matured. (or maybe she was instinctively avoiding a scenario where mating would be encouraged- even though they were neutered and she was spayed)
At this point, she will only occasionally groom one of the boys as he is more eager for her affection. He has learned to approach her by walking by rubbing up against her and then turning around and lowering his head in front of her, and if she is in the mood...she will groom his face and he will lie down with his belly up loving every bit of it, LOL.
The other boy is more independent although he is somewhat protective of her and his siblings. (in regards to interactions with our other non-related cat)
LOL, sorry to go off on a tangent there...it just fascinates me, the inner workings of their relationships. Mainly because this is my first experience with cats who have been raised by their mother to adulthood.
Obviously, these are only my observations of my cats, I am sure the relationships vary widely amongst other feline families.
More to the point, unless you are seeing real aggressiveness (intent to hurt, not warn) with the mother towards the kittens, I wouldn't worry about it. And her milk will gradually diminish as she weans them.
Good luck with your new additions!