If she is pregnant, it will not hurt her to nurse the kittens. I am surprised she is letting them do so.
It would be strange for 3-4 month old kittens to still want to nurse.
You can see if she has milk. Don't laugh at me, but you can grasp the nipple between your finger. Gently squeeze just below the nipple. If you see a clear fluid come out, then that is colostrum which is the first milk with the mother's antibiodies in it. If you get white milk, then she is definitely pregnant. If she has milk, her the mammary tissue, surrounding her nipples will become 3-5 times larger than normal.
Maybe you could put them in the bathroom, with a meal, and then shut the door so Syren can have a break. If she seems to like caring for the kittens, then I would not worry about it. She sounds like a very nice girl.
Also, if she is pregnant, you will want to start her on kitten food for the last month of her pregnancy . Gestation for cats is 60-63 days after breeding.
I hope this helps.
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