Is it ok for a young cat with no kittens to produce milk and to start feeding 12 week old kittens?

Mom cats often adopt another cat's litter, especially if the mom recently gave birth. For young kittens, that's okay BUT mom will need more food with protein (e.g. a good dry and wet food). However, 12-week old kittens should NOT be nursing. Separate the older kittens from their "adopted" mom and get them onto dry food.