I would agree with your sister and PPs that the gaps between meals are probably unreasonable. 7 til 1 is a long time - DS1 has breakfast at 8 and will then have fruit at 11, lunch at 11:40-12 (school) and something else at recess if he has anything left; and then dinner is usually around 6/6:30pm.
DS2 has breakfast at 8ish, then lunch at 11:30-12, depending on whether we're out or not in the morning, and dinner around 6/6:30pm. We all eat dinner together, I can't be doing with having a later meal for adults, and besides DH is usually in bed by 8:30/9pm, so it wouldn't work. He'd be climbing the walls being made to wait that long for food!
DS1 is now of an age where he is "starving"
when he gets home from school - but as he's 22kg wringing wet, at 7, I'm not bothered about him having something to eat then, especially as he has activities after school most days. So he has some bread and butter or fruit, or sometimes a biscuit or bag of crisps to tide him over.
I entirely understand your motivation but I think it is unreasonable of you to impose your really quite stringent regime on guests, especially guests with small children, who will be unused to such large gaps. Of course there is no reason they could not have provided themselves with snacks, but really they wouldn't have expected to, I'm sure!
There have been threads on here before where people have visited ILs and been treated to similarly spartan regimes, and people have been generally horrified at how they were treated.
In future I think it would be a good idea to relax your rules while you have guests - including for your own DDs! - so that people don't go away feeling like they've been treated poorly as guests. I'm not saying that you should put on daily feasts or non-stop food, just have more available for those who are hungry.