Your sister did you a big favour by talking to you in a sensitive way. She needed to say something (and probably should have told you the first night rather than at the end). I appreciate you feel bad now, but I think (without realising it) you treated your guests very harshly and subjected them to a weekend where they were deprived of food, hungry, irritable and powerless to do anything about it, since you live rurally and they were dependent on you for food.
I don't think it's ever ok to impose strict mealtimes, portion control or a no-snacks rule on guests, no matter what you do with your own family in private.
I'm not a big eater (and I've always been very slim), but as a guest I would find your routine and meals very inadequate, my blood-sugar would be low, and although I'd go along with your routine I'd feel unwelcome in your house. Like many people I prefer to eat small amounts regularly. There is nothing wrong with nutritious between-meal snacks, grazing is natural and healthy, it keeps blood-sugar stable and prevents overeating at mealtimes. I think you need to be flexible when you have guests and accommodate different needs. You don't have to eat snacks with them but you do need to make snacks available.
When we have guests I like to offer as much choice and variety as possible with minimum hassle. I always have a well-stocked fruit bowl and bowls of snacks out in living-room (e.g. nuts, wrapped chocolates, cereal bars, biscuit tin etc) so people can help themselves when they feel like it. I also tell guests to help themselves to food from fridge/cupboards and make sure I've stocked up on things like yogurts, cheese, crackers.
Offering cereal as the only breakfast option sounds stingy and controlling. I hate cooking in the morning (and find cooked breakfasts greasy), so when we have guests I get up around 7:30 and put out a selection of cereals, porridge sachets, fresh breads for toasting, variety of spreads (jams, marmalades, butter, soft cheeses, nutella), plate of warmed croissants, and a selection of Danish pastries (normally the mini-pastries selection in a big box from Tesco bakery). This takes minutes to set this up, looks attractive and welcoming, and guests can help themselves as and when they wake up.
Mid-morning around 10:30 I offer hot drinks and cake for anyone who wants it, maybe a tray of toasted tea-cakes or scones for variety.
Lunch would probably be selection of breads, cheeses, cold meats, salad. Timing for meals would depend on when guests were hungry, lunch would usually be around 1-2pm.
Mid-afternoon e.g. 4pm I'd make another round of hot drinks and offer various snacks.
Evening meal I'd either cook for around 7pm or we'd all go out to eat.
IMO having guests is a time to relax your normal rules, get food in that you wouldn't normally buy/eat, and try not to let your own food rules affect guests. I also think it's polite to ask guests in advance what their normal mealtimes are and if they or their kids have any special likes/dislikes. Being a good host is about putting guests first. If you don't want your kids to eat sugary high-calorie snacks in between meals, at the very least offer healthy snacks like fruit, yogurt, granola-bars, crisp breads, rice-cakes, low-fat cheese, salad items etc.