My parents were health nuts - we ate low fat, healthy foods at home - no sweets, no sodas etc.
But on Sundays we went to my Italian Grandmother's house for dinner - things that were battered and fried, pastas, sauces, beef, and cakes and desserts, and sweet tea - the orgy of over abundance that is a Grandmother from the old country.
One day a week, a few holidays, did not ruin us or our healthy eating. In fact, at 47, and a parent for 16 years, I feed my son the way I raised- lots of healthy options with an occasional old world food orgy thrown in.
Your mother, like my Grandmother, shows love through food - Eh, maybe not the best way to go about it - and maybe a result of her own hang ups.
So, easy strategies -
When she shows up with a plate of cookies, take them from her, kiss her cheek, and put them up and away. Explain to her and the kids that they are for later. Repeat as necessary. Will your Mom get mad - maybe - but she will get over it. No more " I was fuming, but gave in because it would have been 4 against 1."
When she makes you a plate to take home from house - take half of the sweets off it before you even walk out of her kitchen.
Take her with you to you daughter's next Dr.s appointment. Let her hear it from the Dr. that your daughter's weight has become a concern.
Look up some of your Mom's favorite recipes and give her a guide to healthier substitutes - many people bake with sweeteners as opposed to sugar, etc. Maybe she can begin baking in lower calorie, healthier ways.
As for the goldfish - My Aunt did that to me with a Beta - Eh, I made my son the sole caretaker of the fish. Poor thing died. So sad, no more fish. Evil, but effective - my son learned to better care for the next one my Aunt gave him. LOL
Good Luck
Hugs.