Some might argue that it is a question of biology. A cat is a communal creature and if it is a part of your family/community it feels responsible for the upbringing of the children in that unit. And while that explanation does go some way to explaining this situation, it fails miserably at explaining the more common cases of cat cruelty towards kids. So we are left with more questions to answer at this point than we began with but that is ok, it is the very reason this parent posted about their unique situation in the first place.
So if you want to find out how and why a cat would suddenly care so much for a creature that does not feed it, does not play nicely with it, does not have seniority over it and in general is not too well suited to cats. We invite you to dive into this sweet tail and see for yourself what it is like to coparent with a cat.