Sounds like your mom is wallowing in a lot of guilt and is trying to navigate her way out, but really floundering on how to do it.

Mom is guilty that she feels that she didn't spend enough time with you, and "neglected" you.

She is able to spend more time with sis, but I suspect she feels that it's still not enough. So she's going to show "everyone" just how much love she can cram into those hours with sis. And one way is to cook fresh food every day, for the (alleged) nutritional value, but also for the visible effort - you can see me cook, my motherly love goes into every stir of the spoon in the pot. See what a good mom I am!

When you question her behavior, you are "accusing" her of not loving sis "the right way" or maybe not enough, because she immediately discredits your comments and accuses you of attacking her.

Maybe a talk with mom, with suggestions of better ways to show her love, by doing things that mom and sis will both enjoy and be able to bond over - rather than spending time cooking all the time, eat occasional leftover and spend the extra time playing a game. Or doing a batch pot cooking of a soup/stew/etc with sis helping so they can bond over occasionally cooking together. But definitely reiterating that she did a good job with you, and sis will be a-ok, especially with additional time bonding with mom.