I don't have 2 kids yet but my thoughts for the early days would be this:
take her some homemade meals for her freezer. Stuff so she doesn't feel pressured to eat it right away, but can easily grab it out her freezer and stick it in the microwave when needed. Could be as simple as a cottage pie or two, or some bolognaise so all she has to do is make the spagghetti. Or any other easy to freeze and reheat meals.
When you go and visit stand and wash dishes, or offer to pack her laundry away. Or maybe say "can I come over for an hour to help you with housework" or something like that.
Not sure how confident the toddler is but maybe offer to take the toddler out to the park for an hour, or even for 30 min, so she can have some time alone at home with the new baby.
Actually, even when she's pregnant that could help, give her some resting time.
Even offer to watch her kids at her house so she can go and have a relaxing bath.
When you go and visit call beforehand and ask if there's anything she needs from the shops. Or even just rock up with a few bags of basic groceries - longlife milk, a loaf of bread, some tea/coffee etc. Chances are she'll be fairly housebound the first few weeks and so won't be able to get the shops as needed. Also maybe make sure she has things like cookies and tea and coffee etc to offer to other guests that might pop around so she doesn't feel embarrassed by not having anything.
I figure laundry, housework (cleaning kitchen and bathrooms), grocery shopping and cooking will be the main areas where she could use the most help. Those things can't be neglected but when you have a new baby they often are. When you then also have a toddler under 2 they are even more neglected. So any way you could help out with any of them would be good.