Anna, you could beat yourself up forever about this, and I hope that you won't.

It truly was an accident and I am sure your Mother-in-law knows that.

I like the idea of giving her a personal card with your handwritten note telling her how awful you feel.

When the time is right and she is ready - perhaps then you can convince her to get another pet of her choice.

I do feel terrible for you it must have been so upsetting.