First of all, hugs.
So glad you are here!
Secondly, you have a newborn. That is always such a beautiful but tough time. Always when I feel at my lowest parenting-wise. The bigger kids are lucky to get the basics like food.
For the fighting I would make sure your 10 yo gets some alone time. Also lots of talking with them about how we can phrase things, how to take turns, apologize, forgive, etc.
For whining it depends on what it is. I ask my kids to try again and start with “mama would you please. . .” It makes them think about asking for a solution rather than just stating a problem and expecting you to fix it.