The taco dish will have to be the meal for that night, of course, because it can't be frozen. I'd make SMALL meals - they are just 2 people, they are exhausted, they aren't out exercising, and they don't want a whole ton of food. Make sure their friends and neighbors aren't doing the same thing - they don't need a fridge full of things that will spoil. Also go for some one-dish meals (or 1 plus a salad) so there aren't a lot of dishes to do. Use disposable containers or things you can give her as a gift, so she doesn't have to think about returning dishes she has washed. In addition to what you've mentioned, I think a nice spinach quiche is light, easy to heat up, and a good source of protein and vegetables in the same meal. A chicken vegetable soup is easy to heat up in just a mugful. Be sure your lasagna and other pasta dishes aren't super heavy but they are easy to freeze and reheat. Your 10 year old is nice to make a dessert, but new moms are often trying to lose weight or just feeling kind of bloated, so she should have realistic expectations about what her aunt and uncle are really going to eat! Your sister should not have "keep niece happy" on her to-do list!
If she is breast feeding, keep away from things that cause gas, for either her or the baby: nothing too spicy, not a lot of broccoli or green peppers or beans. Some women have no problem, but others do.
She may need you more to do other things - run a load of laundry, do the dishes, vacuum, field some telephone calls, sort through gifts & cards to make a list of who sent what and who got a thank you note already, help her learn to do basic tasks like baths, and just go along with her for some "firsts" like first trip to the store, first time figuring out the stroller, and so on. On the side, it's a good lesson for your 10 year old about how much freaking work a baby is, and how they're fussy and can't always be calmed down!