If all the cats are happy and have enough room to give them their own space then there is no reason from that side why you can't keep them. They've grown up with lots of cats around so may quite like the company! If you have the time and resources to make sure all cats get their fair share of attention, play time, fuss time, food, vets etc then again, if you want to keep them there is no reason why you shouldn't.
They are so adorable. I can see why you would have trouble letting them go!!!