Hi Wiggal,
This is absolutely fine, as the kittens are old enough for it not to be an issue. Cats have blood groups, as do we obviously, but in cats there is A, AB, and B types. The only time when kittens would be at risk from feeding off a girl that isn't their mum, is within the first three days of life, when the gut, which is open when they are born, is starting to seal. If an A kitten feeds off a B girl, then this can cause a violent antibody reaction, which sadly can kill the kittens, but at this age it is fine - I currently have a girl feeding five kittens, only two of which are her own, but as they are old enough, it's not a problem. Some girls are just naturally more maternal than others, and will happily keep on feeding the kittens for literally months unless stopped, which I always do, as it really takes it out of the girl.
Hope this helps to reassure you.
Cheers
Plessy