For breakfast, eggs are low calorie.

Serve scrambled, fried, or omelet with veggie or meat of choice

Home fries /wedge potatoes or tater tots are a hit

Bacon

Greek yogurt parfait (blueberries, yogurt, stews berries, chia seeds, honey)

Choose one from below for main entree:
Short ribs
Rack of lamb
Lamb chops or pork chops
Shepherds pie
Roast beef
Ham


Side dishes:
Sautéed zucchini or broccolini in olive oil, garlic

potato au gratin or mashed potatoes

Cauliflower mashed potatoes or cauliflower tater tots for mom


Caesar salad or Greek salad

What do normally serve on Easter and why can't you do that again?

Does Mom like to cook? If so, let her takeover (enjoy relaxing for a bit) or at least help.

Do you need appetizer and dessert ideas too or just main meal?

Are you doing a big brunch prior? If so, maybe go lighter fare for dinner.

Ham
Potatoes
Quiche
Salad
Fruit

For make-ahead, I would think some sort of pot roast or braised beef would be good. Can serve with either mashed potatoes and/or sauteed veggies on the side. A big pot of beef, mushroom and barley soup is hearty an can be made ahead.

Presumably, everyone can eat the meat. So make that whichever way people like. No potatoes for mom, so offer roasted vegetables (roasted root veggies as a separate pan if you go that route). You can prep them ahead of time without salt (I usually do olive oil and garlic) and stick them in the oven, salting right before they go in.

All things egg should be fine for breakfast. You can also do breakfast sausages and bacon. There are low carb breads on the market that are perfectly acceptable substitutes for regular white bread, so mom can have that if she wants toast. You can make breakfast burritos with low carb tortilla instead. Always serve fruit, and as PP said, you can add pastry type things for the kids if you want, as long as mom has options.

My DH is diabetic. He can have all the veggies he wants (but not root vegetables), but not things like bread and pasta. Berries are better options for fruit.

I would get a big old ham, because you can make multiple things out of that, serve it with salad, vegetables, potatoes. You can chop it up and make omelette or quiche with it. Kids can have cinnamon rolls or more brunchy items as well.

Well for diabetes, meat and potatoes (with skin on) is not that bad. No added sugars in the sauce, plenty of whole vegetables and fruits. You can make yogurt parfaits for a special breakfast, or dessert. Yogurt that has no sugar added is good for diabetics. You can cook whole wheat pasta, and see if your father notices it's different (boil for longer). Stock whole grain bread instead of white. I would think about lasagna and shepherd's pie, things like that...

Dad: meat-and-potatoes, hates chicken, likes things very typically American and simple
Mom: has diabetes and is trying to eat healthfully, but can do small portions of just about anything
My family: two adults and two kids, all good eaters

Please help me think of things that are relatively easy and make-ahead (if possible) for dinners. We're going out to a nice dinner on Saturday night with my brother/his partner, so that's "the special Easter meal," but I'll still have to do something decently good for Easter breakfast and dinner.

Ideas, please! Thank you!