Taking the mother cat to the Vet for care was a very kind thing to do.
As the Vet recommended, and I would agree, if you aren't intending to take in all the cats (mother including all kittens), then you must leave the mother out to raise the kittens until they are more grown and can attempt to make it on their own. You may run into issues with this plan though because your neighbors and/or local animal-control officers might collect the cat/kittens before the kittens are more grown. Otherwise you may consider taking them to an animal control facility yourself and see if the facility can get them adopted.
Make certain that the cats are truly stray/feral. One of your
neighbors might just own cats that spend most of their day outside,
and finding that one of their neighbors took them, or reported them to
animal control could cause issues.
Given that you feed them and they appear stray/feral, maybe unclean, you may have already determined for certain that they aren't owned by anyone else. If they are owned by someone else and they are not caring for them, you are in a way morally doing the right thing anyway.
You mention you have a cat already and it doesn't get along with the stray mother cat.
This is problematic because:
- you are causing tension in your home for your existing pet
- you are exposing your existing pet to disease that the mother and/or babies have that you aren't aware of.
- the tension in the home might cause your existing pet to exhibit behavior change that you might find unwelcoming.
If it were me, I would contact a local animal control office and report the mother and kittens for pickup. You are giving them the best shot at being adopted and cared for properly. You are also respecting the nature of your existing cat and not causing undue tension and anxiety.
From your comments, you mention not having animal control or shelters available in your area.
Sounds to me like you have three options:
- Take them both in at the same time, and get care for them from a Vet. Deal with any stress this causes in your home for your existing pet. Know that they might try to escape from your home at any chance they can get.
- Capture both of them and take them to a nearby town/city that does have animal control and see if they can help you.
- Leave them outside, continue to feed them as you have been doing and consider this you doing a good deed for these animals that are in need of a home.