Bless her, if you can keep it clean hopefully it will heal by itself. Keep a close watch on it though, as sometimes secondary infections can be worse than the primary infection.

Also bear in mind that the happy flock of kittens you have now will start reproducing in the very near future and you will end up with hundreds of little feral cats running around Try contacting your nearest cat protection/rescue and tell them you are happy to keep them on your property and feed them, but could they perhaps neuter them at a reasonable price for you. That way you will keep your happy little flock and they won't multiply and take over totally.

Keep up the good work with the abcess