Hi, if it is possible, try to get them sheltered. The kittens
are very dependable on their mother now, if you euthanized their
mom, the kittens will probably follows as well.
Let nature take its course, don't interfere by killing the mum, we
won't know whether the mum will die or not or just feeling signs of
weakness. But what you could do to help is to give them a place
where they can rest, shielded from hot sun or rain and thus
allowing the mum to recuperate.
Try feeding her milk instead, doubt she has the strength to chew or
eat the fish. Start with something light and hope she can
recover.
